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/DMChannel.js

@@ -27,6 +27,10 @@ class DMChannel extends Channel {
      */
     this.recipient = this.client.users.add(data.recipients[0]);
 
+    /**
+     * The ID of the last message in the channel, if one was sent
+     * @type {?Snowflake}
+     */
     this.lastMessageID = data.last_message_id;
   }