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/TextChannel.js

@@ -37,6 +37,10 @@ class TextChannel extends GuildChannel {
      */
     this.nsfw = data.nsfw || /^nsfw(-|$)/.test(this.name);
 
+    /**
+     * The ID of the last message sent in this channel, if one was sent
+     * @type {?Snowflake}
+     */
     this.lastMessageID = data.last_message_id;
 
     if (data.messages) for (const message of data.messages) this.messages.add(message);