feature #6076 added a sentence about the HttpException::setHeaders method (smatyas)

This PR was submitted for the 2.8 branch but it was merged into the master branch instead (closes #6076).

Discussion
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added a sentence about the HttpException::setHeaders method

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes symfony/symfony#17111
| Applies to    | 2.8
| Fixed tickets |

This PR is a doc. change related to the linked symfony PR.

Commits
-------

a5113cf added a sentence about the HttpException::setHeaders method

Author: xabbuh

components/http_kernel/introduction.rst

@@ -560,7 +560,9 @@ below for more details).
        then ``getStatusCode`` and ``getHeaders`` are called on the exception
        and used to populate the headers and status code of the ``FlattenException``
        object. The idea is that these are used in the next step when creating
-       the final response.
+       the final response. If you want to set custom HTTP headers, you can always
+       use the ``setHeaders`` method on exceptions derived from the
+       :class:`Symfony\\Component\\HttpKernel\\Exception\\HttpException` class.
 
     3) A controller is executed and passed the flattened exception. The exact
        controller to render is passed as a constructor argument to this listener.
@@ -595,7 +597,7 @@ each event has their own event object:
 =====================  ================================  ===================================================================================
 Name                   ``KernelEvents`` Constant         Argument passed to the listener
 =====================  ================================  ===================================================================================
-kernel.request         ``KernelEvents::REQUEST``         :class:`Symfony\\Component\\HttpKernel\\Event\\GetResponseEvent`                    
+kernel.request         ``KernelEvents::REQUEST``         :class:`Symfony\\Component\\HttpKernel\\Event\\GetResponseEvent`
 kernel.controller      ``KernelEvents::CONTROLLER``      :class:`Symfony\\Component\\HttpKernel\\Event\\FilterControllerEvent`
 kernel.view            ``KernelEvents::VIEW``            :class:`Symfony\\Component\\HttpKernel\\Event\\GetResponseForControllerResultEvent`
 kernel.response        ``KernelEvents::RESPONSE``        :class:`Symfony\\Component\\HttpKernel\\Event\\FilterResponseEvent`