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/BaseClient.js

@@ -42,6 +42,7 @@ class BaseClient extends EventEmitter {
   /**
    * API shortcut
    * @type {Object}
+   * @readonly
    * @private
    */
   get api() {