minor #15191 [Testing] Every custom header in functional testing client must have HTTP_ prefix (Ferror)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead.

Discussion
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[Testing] Every custom header in functional testing client must have HTTP_ prefix

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Due to symfony/symfony#5074 issue, I would propose to add a simple caution block with information, that the `HTTP_` prefix is mandatory - it will save some hours for others.

Commits
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8e5d9bc Every custom header in testing client must have HTTP prefix

Author: javiereguiluz

testing.rst

@@ -1029,6 +1029,10 @@ You can also override HTTP headers on a per request basis::
         'HTTP_USER_AGENT' => 'MySuperBrowser/1.0',
     ]);
 
+.. caution::
+
+    Every custom header must have `HTTP_` prefix.
+
 .. tip::
 
     The test client is available as a service in the container in the ``test``