docs: add documentation

Accompanying PR to flarum/framework#4474

Author: DavideIadeluca

docs/extend/update-2_0.md

@@ -161,6 +161,9 @@ There have been many changes to the core frontend codebase, including renamed or
 * The `avatar` and `icon` helpers have been refactored to new `Avatar` and `Icon` components. Which now allows you to extend them to modify their behavior.
 * The `Modal` component has been split into `Modal` and `FormModal`. The `Modal` component is now a simple modal that can be used for any content, while the `FormModal` component is a modal that is specifically designed for forms.
 * `app.extensionData` has been removed. You must now use the `Admin` extender to register settings, permissions and custom extension pages.
+* `NicknameModal.prototype` has been moved to `ChangeNicknameModal.prototype` and now requires a `user` prop.
+* `ChangePasswordModal.prototype` has been moved to `RequestPasswordResetModal.prototype` and now requires a `user` prop.
+* The `settings` route has been removed. Settings are now accessible at `user.settings` (`/u/:username/settings`). Extensions which extend the settings page must refactor their implementation to use `this.user`instead of `app.session.user` for fetching and saving settings.
 
 ##### <span class="notable">Notable</span>
 * The admin sidebar navigation has been overhauled. Extension categories are now collapsible groups with count badges and category icons. Categories start collapsed by default, and searching auto-expands categories with matching results. The active extension's category is pre-expanded on page load. Extensions should declare their category in `composer.json` under `extra.flarum-extension.category`. See the [Extension Categories](./admin.md#extension-categories) section in the admin docs for the full list of available categories and how to register custom ones.