docs: Fixed some missing docstrings or incorrect return types (#2093)

* Fix some missing doc strings
Mainly just readonly tags

* Return an error when guild#allowDMs is ran from a bot account, and fix some return types

* WebhookClient implements Webhook, doesn't extend it

* Fix Client#rateLimit docs not showing what it returns

Cause I wanted to handle this event only to see no return props 🤔

* Actually make Client#rateLimit show the right info

Its an object with all the info

src/client/WebhookClient.js

@@ -3,7 +3,7 @@ const BaseClient = require('./BaseClient');
 
 /**
  * The webhook client.
- * @extends {Webhook}
+ * @implements {Webhook}
  * @extends {BaseClient}
  */
 class WebhookClient extends BaseClient {