Fix some documentation about the category channels (#1917)

* Document CategoryChannel

* More fixes

* Sorry Crawl

* createChannel can return CategoryChannel

* Did senks requests

* I can't grammar

* Other snek request

src/structures/CategoryChannel.js

@@ -1,6 +1,15 @@
 const GuildChannel = require('./GuildChannel');
 
+/**
+ * Represents a guild category channel on Discord.
+ * @extends {GuildChannel}
+ */
 class CategoryChannel extends GuildChannel {
+  /**
+   * The channels that are part of this category
+   * @type {?Collection}
+   * @readonly
+   */
   get children() {
     return this.guild.channels.filter(c => c.parentID === this.id);
   }