minor #8180 Added a caution note about UploadedFile and insulated tests (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Added a caution note about UploadedFile and insulated tests

This will help avoid issues like symfony/symfony#19572.

Commits
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eacd844 Added a caution note about UploadedFile and insulated tests

Author: xabbuh

testing/insulating_clients.rst

@@ -43,3 +43,11 @@ clean PHP process, thus avoiding any side-effects.
 
     As an insulated client is slower, you can keep one client in the main
     process, and insulate the other ones.
+
+.. caution::
+
+    Insulating tests requires some serializing and unserializing operations. If
+    your test includes data that can't be serialized, such as file streams when
+    using the ``UploadedFile`` class, you'll see an exception about
+    *"serialization is not allowed"*. This is a technical limitation of PHP, so
+    the only solution is to disable insulation for those tests.