docs: document lastMessageID for TextChannel and (Group)DMChannel (#2602)

* Adds JSDocs for TextChannel.lastMessageID

* oops

* docs: document lastMessageID in (Group)DMChannel as well

src/structures/GroupDMChannel.js

@@ -98,6 +98,10 @@ class GroupDMChannel extends Channel {
       }
     }
 
+    /**
+     * The ID of the last message in the channel, if one was sent
+     * @type {?Snowflake}
+     */
     this.lastMessageID = data.last_message_id;
   }