[DOCS] Improve LoginProviderInterface->modifyView() ReST (#5113)

* [DOCS] Improve LoginProviderInterface->modifyView() ReST

Resolves: TYPO3-Documentation/Changelog-To-Doc#1056
Releases: main, 13.4

* Apply suggestions from code review

Co-authored-by: Stefan Frömken <froemken@gmail.com>

---------

Co-authored-by: Stefan Frömken <froemken@gmail.com>

Author: linawolf

Documentation/ApiOverview/Backend/LoginProvider.rst

@@ -89,6 +89,16 @@ An implementation of :php-short:`\TYPO3\CMS\Backend\LoginProvider\LoginProviderI
 look like this for TYPO3 v13:
 
 ..  literalinclude:: _LoginProvider/_MyLoginProvider.php
+    :caption: EXT:my_extension/Classes/Login/MyLoginProvider.php
+    :linenos:
+
+The default implementation in :php-short:`\TYPO3\CMS\Backend\LoginProvider\UsernamePasswordLoginProvider`
+is a good example. Extensions that need to configure additional template, layout or
+partial lookup paths can extend them, see lines 23-28 in the example above.
+
+Consumers of :php-short:`\TYPO3\CMS\Backend\LoginProvider\Event\ModifyPageLayoutOnLoginProviderSelectionEvent`
+should use the request instead, and/or should get an instance of
+:php-short:`\TYPO3\CMS\Core\Page\PageRenderer` injected as well.
 
 ..  _login-provider-view:
 

Documentation/ApiOverview/Backend/_LoginProvider/_MyLoginProvider.php

@@ -7,6 +7,7 @@
 use TYPO3\CMS\Backend\LoginProvider\LoginProviderInterface;
 use TYPO3\CMS\Core\Page\PageRenderer;
 use TYPO3\CMS\Core\View\ViewInterface;
+use TYPO3\CMS\Fluid\View\FluidViewAdapter;
 
 #[Autoconfigure(public: true)]
 final readonly class MyLoginProvider implements LoginProviderInterface
@@ -18,6 +19,13 @@ public function __construct(
     public function modifyView(ServerRequestInterface $request, ViewInterface $view): string
     {
         $this->pageRenderer->addJsFile('someFile');
+        // Custom login provider implementations can add custom fluid lookup paths.
+        if ($view instanceof FluidViewAdapter) {
+            $templatePaths = $view->getRenderingContext()->getTemplatePaths();
+            $templateRootPaths = $templatePaths->getTemplateRootPaths();
+            $templateRootPaths[] = 'EXT:my_extension/Resources/Private/Templates';
+            $templatePaths->setTemplateRootPaths($templateRootPaths);
+        }
         $view->assign('Some Variable', 'some value');
         return 'Login/MyLoginForm';
     }