minor #8218 Added a note about the new behavior of BrowserKit and redirects (javiereguiluz)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #8218).

Discussion
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Added a note about the new behavior of BrowserKit and redirects

This fixes #8164.

I couldn't find any mention to the old behavior, but I consider important to add a note about this to help users upgrading to newer Symfony versions and facing failing tests.

Commits
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48553b7 Added a note about the new behavior of BrowserKit and redirects

Author: xabbuh

testing.rst

@@ -421,6 +421,11 @@ The Client supports many operations that can be done in a real browser::
     // Clears all cookies and the history
     $client->restart();
 
+.. versionadded:: 3.4
+    Starting from Symfony 3.4, the ``back()`` and ``forward()`` methods skip the
+    redirects that may have occurred when requesting a URL, as normal browsers
+    do. In previous Symfony versions they weren't skipped.
+
 Accessing Internal Objects
 ~~~~~~~~~~~~~~~~~~~~~~~~~~