[skip ci]Add rmq consumer impl MsgMutex comment (#11532)

Signed-off-by: fishpenguin <kun.yu@zilliz.com>

internal/util/rocksmq/client/rocksmq/consumer_impl.go

@@ -94,6 +94,7 @@ func (c *consumer) Topic() string {
 	return c.topic
 }
 
+// MsgMutex return the message mutex channel of consumer
 func (c *consumer) MsgMutex() chan struct{} {
 	return c.msgMutex
 }