Added and fixed a bunch of docs stuff (#1767)

- Fixed a common copy paste fail `the the <thing>` in various places
- Apparently I can't type Resolvable correctly,
Fixed that wherever applicable
- Documented GroupDMChannel#nicks so that it will be displayed on the docs
- GroupDMChannel#icon is nullable
- Removed empty InviteOptions typdef, as its properties are now documented in GuildChannel#createInvite
- MessageMentions#channels is no longer nullable
- RoleData#permissions takes a PermissionResolvable or an array of them
- Webhook#avatar is nullable
- Added HTTPOptions typedef and added it to ClientOptions typedef
- ClientUserChannelOverride#muted is for a channel and not a guild directly

src/structures/Webhook.js

@@ -40,7 +40,7 @@ class Webhook {
 
     /**
      * The avatar for the webhook
-     * @type {string}
+     * @type {?string}
      */
     this.avatar = data.avatar;