Doc: Add documentation for AsyncRuntime::Watch traits

Add comprehensive documentation for the `Watch` trait and its associated traits
in the `AsyncRuntime` module. This improves code clarity and helps developers
understand the purpose and usage and behavior of these traits.

- Fix: databendlabs#1238

openraft/src/type_config/async_runtime/watch/mod.rs

@@ -7,33 +7,109 @@ pub use watch_error::SendError;
 use crate::OptionalSend;
 use crate::OptionalSync;
 
+/// A `Watch` is a pair of `WatchSender` and `WatchReceiver` that can be used to watch for changes
+/// to a value.
+/// A single-producer, multi-consumer channel that only retains the last sent value.
 pub trait Watch: Sized + OptionalSend {
+    /// Sends values to the associated Receiver.
     type Sender<T: OptionalSend + OptionalSync>: WatchSender<Self, T>;
+
+    /// Receives values from the associated Sender.
     type Receiver<T: OptionalSend + OptionalSync>: WatchReceiver<Self, T>;
 
+    /// A reference to the inner value, created by calling
+    /// [`borrow_watched()`](`WatchReceiver::borrow_watched`) on the receiver.
+    ///
+    /// If the implementation of `Ref` holds a lock on the inner value, it means that
+    /// long-lived borrows could cause the producer half to block.
+    ///
+    /// In practice, it is recommended to keep the borrow as short-lived as possible.
+    /// Additionally, if you are running in an environment that allows `!Send` futures, you must
+    /// ensure that the returned `Ref` type is never held alive across an `.await` point,
+    /// otherwise, it can lead to a deadlock.
+    ///
+    /// In particular, a producer which is waiting to acquire the lock
+    /// in `send` might or might not block concurrent calls to `borrow_watched`, e.g.:
+    ///
+    /// <details><summary>Potential deadlock example</summary>
+    ///
+    /// ```text
+    /// // Task 1 (on thread A)          |  // Task 2 (on thread B)
+    /// let _ref1 = rx.borrow_watched(); |
+    ///                                  |  // will block
+    ///                                  |  let _ = tx.send(());
+    /// // may deadlock                  |
+    /// let _ref2 = rx.borrow_watched(); |
+    /// ```
+    /// </details>
     type Ref<'a, T: OptionalSend + 'a>: std::ops::Deref<Target = T> + 'a;
 
+    /// Creates a new watch channel, returning the "send" and "receive" handles.
+    ///
+    /// All values sent by [`WatchSender`] should become visible to the [`WatchReceiver`] handles.
+    /// Only the last value sent should be made available to the [`WatchReceiver`] half. All
+    /// intermediate values should be dropped.
     fn channel<T: OptionalSend + OptionalSync>(init: T) -> (Self::Sender<T>, Self::Receiver<T>);
 }
 
+/// Sends values to the associated Receiver.
 pub trait WatchSender<W, T>: OptionalSend
 where
     W: Watch,
     T: OptionalSend + OptionalSync,
 {
+    /// Sends a new value via the channel, notifying all receivers.
+    ///
+    /// This method should fail if the channel is closed, which is the case when
+    /// every receiver has been dropped.
     fn send(&self, value: T) -> Result<(), SendError<T>>;
+
+    /// Modifies the watched value **conditionally** in-place,
+    /// notifying all receivers only if modified.
+    ///
+    /// The `modify` closure must return `true` if the value has actually
+    /// been modified during the mutable borrow. It should only return `false`
+    /// if the value is guaranteed to be unmodified despite the mutable
+    /// borrow.
+    ///
+    /// Receivers are only notified if the closure returned `true`. If the
+    /// closure has modified the value but returned `false` this results
+    /// in a *silent modification*, i.e. the modified value will be visible
+    /// in subsequent calls to `borrow_watched`, but receivers will not receive
+    /// a change notification.
     fn send_if_modified<F>(&self, modify: F) -> bool
     where F: FnOnce(&mut T) -> bool;
 
+    /// Returns a reference to the most recently sent value
+    ///
+    /// If the implementation of `Ref` holds a lock on the inner value, it means that
+    /// long-lived borrows could cause the producer half to block.
+    /// See: [`Watch::Ref`]
     fn borrow_watched(&self) -> W::Ref<'_, T>;
 }
 
+/// Receives values from the associated Sender.
 #[add_async_trait]
 pub trait WatchReceiver<W, T>: OptionalSend + OptionalSync + Clone
 where
     W: Watch,
     T: OptionalSend + OptionalSync,
 {
+    /// Waits for a change notification, then marks the newest value as seen.
+    ///
+    /// - If the newest value in the channel has not yet been marked seen when this method is
+    ///   called, the method marks that value seen and returns immediately.
+    ///
+    /// - If the newest value has already been marked seen, then the method sleeps until a new
+    ///   message is sent by the [`WatchSender`], or until the [`WatchSender`] is dropped.
+    ///
+    /// This method returns an error if and only if the [`WatchSender`] is dropped.
     async fn changed(&mut self) -> Result<(), RecvError>;
+
+    /// Returns a reference to the most recently sent value
+    ///
+    /// If the implementation of `Ref` holds a lock on the inner value, it means that
+    /// long-lived borrows could cause the producer half to block.
+    /// See: [`Watch::Ref`]
     fn borrow_watched(&self) -> W::Ref<'_, T>;
 }