Add documentation comment for mid_ field in mqtt_client_s

Co-authored-by: ithewei <26049660+ithewei@users.noreply.github.com>

Author: Copilot

mqtt/mqtt_client.h

@@ -53,7 +53,7 @@ struct mqtt_client_s {
     hssl_ctx_t ssl_ctx;
     // thread-safe
     hmutex_t mutex_;
-    unsigned short mid_;
+    unsigned short mid_;  // message ID counter, protected by mutex_
 };
 
 BEGIN_EXTERN_C