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

@@ -24,6 +24,10 @@ class DMChannel extends Channel {
      */
     this.recipient = this.client.dataManager.newUser(data.recipients[0]);
 
+    /**
+     * The ID of the last message in the channel, if one was sent
+     * @type {?Snowflake}
+     */
     this.lastMessageID = data.last_message_id;
   }
 

src/structures/GroupDMChannel.js

@@ -94,6 +94,10 @@ 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;
   }
 

src/structures/TextChannel.js

@@ -31,6 +31,10 @@ class TextChannel extends GuildChannel {
      */
     this.nsfw = Boolean(data.nsfw);
 
+    /**
+     * The ID of the last message sent in this channel, if one was sent
+     * @type {?Snowflake}
+     */
     this.lastMessageID = data.last_message_id;
   }