[Documentation] Added a warn in fetchMentions (#1991)

src/structures/ClientUser.js

@@ -245,6 +245,7 @@ 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 {number} [options.limit=25] Maximum number of mentions to retrieve
    * @param {boolean} [options.roles=true] Whether to include role mentions