docs: document the messages property for all TextBasedChannels (#2373)

* Document TextChannel#messages

* Same for DMChannels

* And GroupDMChannels

src/structures/GroupDMChannel.js

@@ -34,6 +34,10 @@ const MessageStore = require('../stores/MessageStore');
 class GroupDMChannel extends Channel {
   constructor(client, data) {
     super(client, data);
+    /**
+     * A collection containing the messages sent to this channel
+     * @type {MessageStore<Snowflake, Message>}
+     */
     this.messages = new MessageStore(this);
     this._typing = new Map();
   }