fix: clarify behavior of agg funcs regarding to nulls

sql/functions/aggregate.mdx

@@ -9,18 +9,17 @@ For details about the supported syntaxes of aggregate expressions, see [Aggregat
 
 ### `array_agg`
 
-Returns an array from input values in which each value in the set is assigned to an array element. The `ORDER BY` clause is optional and specifies the order of rows processed in the aggregation, which determines the order of the elements in the result array.
-
-```bash
-array_agg ( expression [ ORDER BY [ sort_expression { ASC | DESC } ] ] ) -> output_array
+Collects all the input values, including nulls, into an array. The `ORDER BY` clause is optional and specifies the order of rows processed in the aggregation, which determines the order of the elements in the result array.
 
+```sql
+array_agg ( expression [ ORDER BY sort_expression ] ) -> output_array
 ```
 
 ### `avg`
 
-Returns the average (arithmetic mean) of the selected values.
+Returns the average (arithmetic mean) of all non-null input values or null if no non-null values are provided.
 
-```bash
+```sql
 avg ( expression ) -> see description
 ```
 
@@ -33,92 +32,101 @@ Return type is numeric for integer inputs and double precision for float point i
 
 Returns the bitwise AND of all non-null input values or null if no non-null values are provided.
 
-```bash
-bit_and ( smallint, int, or bigint ) -> same as input type
+```sql
+bit_and ( smallint | int | bigint ) -> same as input type
 ```
 
 ### `bit_or`
 
 Returns the bitwise OR of all non-null input values or null if no non-null values are provided.
 
 ```sql
-bit_or ( smallint, int, or bigint ) -> same as input type
+bit_or ( smallint | int | bigint ) -> same as input type
 ```
 
 ### `bool_and`
-Returns true if all input values are true, otherwise false.
+
+Returns true if all non-null input values are true, otherwise false.
 
 ```sql
 bool_and ( boolean ) -> boolean
 ```
 
 ### `bool_or`
 
-Returns true if at least one input value is true, otherwise false.
+Returns true if any non-null input value is true, otherwise false.
 
 ```sql
 bool_or ( boolean ) -> boolean
 ```
 
 ### `count`
 
-Returns the number of non-null rows.
+Returns the number of non-null input values.
 
-```bash
+```sql
 count ( expression ) -> bigint
 ```
 
 The input can be of any supported data type.
 
+### `count(*)`
+
+Returns the number of rows in the input.
+
+```sql
+count(*) -> bigint
+```
+
 ### `jsonb_agg`
 
-Aggregates values, including nulls, as a JSON array. The `ORDER BY` clause is optional and specifies the order of rows processed in the aggregation, which determines the order of the elements in the result array.
+Collects all the input values, including nulls, into a JSON array. The `ORDER BY` clause is optional and specifies the order of rows processed in the aggregation, which determines the order of the elements in the result array.
 
-```bash
-jsonb_agg ( any_element ) -> jsonb
+```sql
+jsonb_agg ( any_element [ ORDER BY sort_expression ] ) -> jsonb
 ```
 
 ### `jsonb_object_agg`
 
-Aggregates name/value pairs as a JSON object.
+Aggregates name/value pairs as a JSON object. Values can be null, but keys cannot.
 
-```bash
-jsonb_object_agg ( key "string" , value "any" ) -> jsonb
+```sql
+jsonb_object_agg ( key "text" , value "any" ) -> jsonb
 ```
 
 ### `max`
 
-Returns the maximum value in a set of values.
+Returns the maximum of the non-null input values, or null if no non-null values are provided.
 
-```bash
+```sql
 max ( expression ) -> same as input type
 ```
 
 Input can be of any numeric, string, date/time, or interval type, or an array of these types.
 
 ### `min`
 
-Returns the minimum value in a set of values.
+Returns the minimum value of the non-null input values, or null if no non-null values are provided.
 
-```bash
+```sql
 min ( expression ) -> same as input type
 ```
 
 Input can be of any numeric, string, date/time, or interval type, or an array of these types.
 
 ### `string_agg`
 
-Combines non-null values into a string, separated by `delimiter_string`. The `ORDER BY` clause is optional and specifies the order of rows processed in the aggregation, which determines the order of the elements in the result array.
+Concatenates non-null input values into a string. Each value after the first is preceded by the corresponding delimiter (if it's not null). If no non-null values are provided, returns null. The `ORDER BY` clause is optional and specifies the order of rows processed in the aggregation, which determines the order of the elements in the result array.
 
-```bash
-string_agg ( expression, delimiter_string ) -> output_string
+```sql
+string_agg ( value text, delimiter text [ ORDER BY sort_expression ] ) -> output_string
 ```
 
 ### `sum`
 
-Returns the sum of all input values.
+Returns the sum of all non-null input values, or null if no non-null values are provided.
 
-```bash
+```sql
 sum ( expression )
 ```
 
@@ -128,19 +136,19 @@ Return type is bigint for smallint or int inputs, numeric for bigint inputs, oth
 
 ### `first_value`
 
-Returns the first value in an ordered set of values.
+Returns the first value in an ordered set of values, including nulls.
 
-```bash
+```sql
 first_value ( expression ORDER BY order_key ) -> same as input type
 ```
 
 `order_key` is the column or expression used to determine the order of the values. It is required to make the result deterministic.
 
 ### `last_value`
 
-Returns the last value in an ordered set of values.
+Returns the last value in an ordered set of values, including nulls.
 
-```bash
+```sql
 last_value ( expression ORDER BY order_key ) -> same as input type
 ```
 
@@ -150,31 +158,31 @@ last_value ( expression ORDER BY order_key ) -> same as input type
 
 Calculates the population standard deviation of the input values. Returns `NULL` if the input contains no non-null values.
 
-```bash
+```sql
 stddev_pop ( expression ) -> output_value
 ```
 
 ### `stddev_samp`
 
 Calculates the sample standard deviation of the input values. Returns `NULL` if the input contains fewer than two non-null values.
 
-```bash
+```sql
 stddev_samp ( expression ) -> output_value
 ```
 
 ### `var_pop`
 
 Calculates the population variance of the input values. Returns `NULL` if the input contains no non-null values.
 
-```bash
+```sql
 var_pop ( expression ) -> output_value
 ```
 
 ### `var_samp`
 
 Calculates the sample variance of the input values. Returns `NULL` if the input contains fewer than two non-null values.
 
-```bash
+```sql
 var_samp ( expression ) -> output_value
 ```
 
@@ -188,7 +196,7 @@ At present, ordered-set aggregate functions support only constant fraction argum
 Computes the mode, which is the most frequent value of the aggregated argument. If there are multiple equally-frequent values, it arbitrarily chooses the first one.
 
 ```sql
-mode () WITHIN GROUP ( ORDER BY sort_expression anyelement ) -> same as sort_expression
+mode () WITHIN GROUP ( ORDER BY sort_expression ) -> same as sort_expression
 ```
 
 `sort_expression`: Must be of a sortable type.
@@ -207,7 +215,7 @@ At present, `percentile_cont` is not supported for [streaming queries](/docs/cur
 
 Computes the continuous percentile, which is a value corresponding to the specified fraction within the ordered set of aggregated argument values. It can interpolate between adjacent input items if needed.
 
-```bash
+```sql
 percentile_cont ( fraction double precision ) WITHIN GROUP ( ORDER BY sort_expression double precision ) -> double precision
 
 ```
@@ -216,14 +224,15 @@ percentile_cont ( fraction double precision ) WITHIN GROUP ( ORDER BY sort_expre
 
 This example calculates the median (50th percentile) of the values in `column1` from `table1`.
 
-```bash
+```sql
 SELECT percentile_cont(0.5) WITHIN GROUP (ORDER BY column1) FROM table1;
 ```
 
 If NULL is provided, the function will not calculate a specific percentile and return NULL instead.
 
 
 ### `percentile_disc`
+
 <Note>
 At present, `percentile_disc` is not supported for streaming queries yet.
 </Note>
@@ -279,7 +288,7 @@ Grouping operation functions are used in conjunction with grouping sets to disti
 
 Returns a bit mask indicating which `GROUP BY` expressions are not included in the current grouping set. Bits are assigned with the rightmost argument corresponding to the least-significant bit; each bit is 0 if the corresponding expression is included in the grouping criteria of the grouping set generating the current result row, and 1 if it is not included.
 
-```bash Syntax
+```sql Syntax
 grouping ( group_by_expression(s) ) → integer
 ```