analysis/lint: don't print so many newlines for description

The Before and After had extraneous newlines; before:

    % staticcheck -explain S1000
    Use plain channel send or receive instead of single-case select

    Select statements with a single case can be replaced with a simple
    send or receive.

    Before:

        select {
        case x := <-ch:
            fmt.Println(x)
        }

    After:

        x := <-ch
        fmt.Println(x)

    Available since
        2017.1

    Online documentation
        https://staticcheck.dev/docs/checks#S1000

And after:

    % staticcheck -explain S1000
    Use plain channel send or receive instead of single-case select

    Select statements with a single case can be replaced with a simple
    send or receive.

    Before:

        select {
        case x := <-ch:
            fmt.Println(x)
        }

    After:

        x := <-ch
        fmt.Println(x)

    Available since
        2017.1

    Online documentation
        https://staticcheck.dev/docs/checks#S1000

analysis/lint/lint.go

@@ -167,13 +167,13 @@ func (doc *Documentation) format(markdown bool, metadata bool) string {
 	if doc.Before != "" {
 		fmt.Fprintln(b, "Before:")
 		fmt.Fprintln(b, "")
-		for _, line := range strings.Split(doc.Before, "\n") {
+		for _, line := range strings.Split(strings.TrimSpace(doc.Before), "\n") {
 			fmt.Fprint(b, "    ", line, "\n")
 		}
 		fmt.Fprintln(b, "")
 		fmt.Fprintln(b, "After:")
 		fmt.Fprintln(b, "")
-		for _, line := range strings.Split(doc.After, "\n") {
+		for _, line := range strings.Split(strings.TrimSpace(doc.After), "\n") {
 			fmt.Fprint(b, "    ", line, "\n")
 		}
 		fmt.Fprintln(b, "")