Add note about g_free transaction after calling push_event in plugin …

…interface documentation

src/plugins/plugin.h

@@ -357,6 +357,7 @@ struct janus_callbacks {
 	 * @note The Janus core increases the references to both the message and jsep
 	 * json_t objects. This means that you'll have to decrease your own
 	 * reference yourself with a \c json_decref after calling push_event.
+	 * You'll also have to free transaction with \c g_free.
 	 * @param[in] handle The plugin/gateway session used for this peer
 	 * @param[in] plugin The plugin instance that is sending the message/event
 	 * @param[in] transaction The transaction identifier this message refers to