add DMChannel docs

src/structures/DMChannel.js

@@ -3,6 +3,11 @@ const TextBasedChannel = require('./interface/TextBasedChannel');
 const User = require('./User');
 const TextChannelDataStore = require('./datastore/TextChannelDataStore');
 
+/**
+ * Represents a Direct Message Channel between two users.
+ * @extends {Channel}
+ * @implements {TextBasedChannel}
+ */
 class DMChannel extends Channel {
   constructor(client, data) {
     super(client, data);
@@ -26,13 +31,34 @@ class DMChannel extends Channel {
 
   setup(data) {
     super.setup(data);
+    /**
+     * The recipient on the other end of the DM
+     * @type {User}
+     */
     this.recipient = this.client.store.add('users', new User(this.client, data.recipients[0]));
+    /**
+     * The ID of the last sent message, if available
+     * @type {?String}
+     */
     this.lastMessageID = data.last_message_id;
   }
 
+  /**
+   * When concatenated with a String, this automatically concatenates the recipient's mention instead of the
+   * DM channel object.
+   * @returns {String}
+   */
   toString() {
     return this.recipient.toString();
   }
+
+  sendMessage() {
+    return;
+  }
+
+  sendTTSMessage() {
+    return;
+  }
 }
 
 TextBasedChannel.applyToClass(DMChannel);