minor #12380 Clarify target path functionality (javiereguiluz)

This PR was merged into the 4.3 branch.

Discussion
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Clarify target path functionality

I tried to merge #11192 but I can't because the original fork/branch was removed. This PR reproduces the original PR with the reviewers suggestions applied, so all credit goes to them. Thanks.

Commits
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45f0ddb Clarify target path functionality

Author: javiereguiluz

security/form_login_setup.rst

@@ -423,4 +423,49 @@ deal with this low level session variable. However, the
 :class:`Symfony\\Component\\Security\\Http\\Util\\TargetPathTrait` utility
 can be used to read (like in the example above) or set this value manually.
 
+When the user tries to access a restricted page, they are being redirected to
+the login page. At that point target path will be set. After a successful login,
+the user will be redirected to this previously set target path.
+
+If you also want to apply this behavior to public pages, you can create an
+:doc:`event subscriber </event_dispatcher>` to set the target path manually
+whenever the user browses a page::
+
+    namespace App\EventSubscriber;
+
+    use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+    use Symfony\Component\HttpFoundation\Session\SessionInterface;
+    use Symfony\Component\HttpKernel\Event\GetResponseEvent;
+    use Symfony\Component\HttpKernel\KernelEvents;
+    use Symfony\Component\Security\Http\Util\TargetPathTrait;
+
+    class RequestSubscriber implements EventSubscriberInterface
+    {
+        use TargetPathTrait;
+
+        private $session;
+
+        public function __construct(SessionInterface $session)
+        {
+            $this->session = $session;
+        }
+
+        public function onKernelRequest(GetResponseEvent $event): void
+        {
+            $request = $event->getRequest();
+            if (!$event->isMasterRequest() || $request->isXmlHttpRequest()) {
+                return;
+            }
+
+            $this->saveTargetPath($this->session, 'main', $request->getUri());
+        }
+
+        public static function getSubscribedEvents()
+        {
+            return [
+                KernelEvents::REQUEST => ['onKernelRequest']
+            ];
+        }
+    }
+
 .. _`MakerBundle`: https://symfony.com/doc/current/bundles/SymfonyMakerBundle/index.html