docs(ClientUser): mark email field as user account only property (#2306)

* Store Mail - User Account Only

Added that info

* docs(ClientUser): mark email field as nullable

src/structures/ClientUser.js

@@ -22,7 +22,8 @@ class ClientUser extends Structures.get('User') {
 
     /**
      * The email of this account
-     * @type {string}
+     * <warn>This is only filled when using a user account.</warn>
+     * @type {?string}
      */
     this.email = data.email;
     this._typing = new Map();