clientv3: document possible memory pile-up in Watch

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

clientv3/watch.go

@@ -48,6 +48,8 @@ type Watcher interface {
 	// client will post a compacted error watch response, and the channel will close.
 	// If the context "ctx" is canceled or timed out, returned "WatchChan" is closed,
 	// and "WatchResponse" from this closed channel has zero events and nil "Err()".
+	// The context "ctx" MUST be canceled, as soon as watcher is no longer being used,
+	// to release the associated resources.
 	// If the context is "context.Background/TODO", returned "WatchChan" will not be closed
 	// and wait until events happen, except when server returns a non-recoverable error.
 	// For example, when context passed with "WithRequireLeader" and the connected server