docs/fix(setParent): docs update and nullable channel param (#2160)

* fix(setParent): no longer in GuildChannel

* refactored

* little bit less ugly

* space/appel suggestion

* docs fix

* shhhhhhhh

* fun docs trip

* prototype thing

* mark nullable

src/structures/CategoryChannel.js

@@ -13,6 +13,19 @@ class CategoryChannel extends GuildChannel {
   get children() {
     return this.guild.channels.filter(c => c.parentID === this.id);
   }
+
+  /**
+   * Sets the category parent of this channel.
+   * <warn>It is not currently possible to set the parent of a CategoryChannel.</warn>
+   * @method setParent
+   * @memberof CategoryChannel
+   * @instance
+   * @param {?GuildChannel|Snowflake} channel Parent channel
+   * @param {Object} [options={}] Options to pass
+   * @param {boolean} [options.lockPermissions=true] Lock the permissions to what the parent's permissions are
+   * @param {string} [options.reason] Reason for modifying the parent of this channel
+   * @returns {Promise<GuildChannel>}
+   */
 }
 
 module.exports = CategoryChannel;