docs: add missing docstring for Client#userUpdate (#2930)

* docs: add missing docstring for Client#userUpdate

* docs: indentn't

* docs: indentn't 2

src/client/actions/UserUpdate.js

@@ -9,6 +9,12 @@ class UserUpdateAction extends Action {
     const oldUser = newUser._update(data.user);
 
     if (!oldUser.equals(newUser)) {
+      /**
+       * Emitted whenever a user's details (e.g. username) are changed.
+       * @event Client#userUpdate
+       * @param {User} oldUser The user before the update
+       * @param {User} newUser The user after the update
+       */
       client.emit(Events.USER_UPDATE, oldUser, newUser);
       return {
         old: oldUser,