minor #9692 WDT following AJAX requests (jeffreymb)

This PR was squashed before being merged into the master branch (closes #9692).

Discussion
----------

WDT following AJAX requests

This PR is for #8409 and the functionality in symfony/symfony#26655.

This is my first Doc PR so I'm happy for all the constrictive input there is to give. I'm really hoping this functionality can make 4.1.

Commits
-------

1bbca83 WDT following AJAX requests

Author: javiereguiluz

profiler.rst

@@ -21,3 +21,4 @@ install the profiler before using it:
     profiler/profiling_data
     profiler/matchers
     profiler/storage
+    profiler/wdt_follow_ajax

profiler/wdt_follow_ajax.rst

@@ -0,0 +1,40 @@
+.. index::
+    single: Profiling: WDT Auto-update after AJAX Request
+
+How to Make the Web Debug Toolbar Auto-update After AJAX Requests
+================================================================
+
+For single page applications it would be more convenient if the toolbar 
+showed the information for the most recent AJAX request instead of the 
+initial page load.
+
+By setting the ``Symfony-Debug-Toolbar-Replace`` header to a value of ``1`` in the
+AJAX request, the toolbar will be automatically reloaded for the request. The
+header can be set on the response object::
+
+    $response->headers->set('Symfony-Debug-Toolbar-Replace', 1);
+
+Only Setting the Header During Development
+-------------------------------------------
+
+Ideally this header should only be set during development and not for
+production. This can be accomplished by setting the header in a
+:ref:`kernel.response <component-http-kernel-kernel-response>` event listener::
+
+    public function onKernelResponse(FilterResponseEvent $event)
+    {
+        $response = $event->getResponse();
+
+        $response->headers->set('Symfony-Debug-Toolbar-Replace', 1);
+    }
+
+.. seealso::
+
+    Read more Symfony events :ref:`/reference/events`.
+
+If you are using Symfony Flex, you should define your event listener service in the 
+``config/services_dev.yml`` file so that it only exists in the ``dev`` environment.
+
+.. seealso::
+
+    Read more on creating dev only services :ref:`/configuration/configuration_organization`.