minor #6613 Clarify documentation on serving files (raphaelm)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #6613).

Discussion
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Clarify documentation on serving files

For a beginner, the "Serving Files" documentation was a little bit ambiguous on whether the makeDisposition method only adds the header or deals with sending the content as well. This change tries to make that explicit.

Commits
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7746e07 Clarify documentation on serving files

components/http_foundation/introduction.rst

@@ -490,6 +490,9 @@ abstracts the hard work behind a simple API::
 
     $response->headers->set('Content-Disposition', $d);
 
+Then you must set the file's content as usual, for example by using
+:method:`Symfony\\Component\\HttpFoundation\\Response::setContent`.
+
 Alternatively, if you are serving a static file, you can use a
 :class:`Symfony\\Component\\HttpFoundation\\BinaryFileResponse`::