Improve documentation.

Author: levlam

td/generate/scheme/td_api.tl

@@ -11900,7 +11900,7 @@ deleteChatReplyMarkup chat_id:int53 message_id:int53 = Ok;
 
 //@description Sends a notification about user activity in a chat
 //@chat_id Chat identifier
-//@topic_id Identifier of the topic in which the action is performed
+//@topic_id Identifier of the topic in which the action is performed; pass null if none
 //@business_connection_id Unique identifier of business connection on behalf of which to send the request; for bots only
 //@action The action description; pass null to cancel the currently active action
 sendChatAction chat_id:int53 topic_id:MessageTopic business_connection_id:string action:ChatAction = Ok;