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/structures/ClientUser.js

@@ -248,7 +248,7 @@ class ClientUser extends User {
   /**
    * Fetches messages that mentioned the client's user.
    * <warn>This is only available when using a user account.</warn>
-   * @param {Object} [options] Options for the fetch
+   * @param {Object} [options={}] Options for the fetch
    * @param {number} [options.limit=25] Maximum number of mentions to retrieve
    * @param {boolean} [options.roles=true] Whether to include role mentions
    * @param {boolean} [options.everyone=true] Whether to include everyone/here mentions