minor #16052 Add caution on the framework kernel secret (noniagriconomie)

This PR was merged into the 4.4 branch.

Discussion
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Add caution on the framework kernel secret

Hi, small caution on the doc according to this framework config value, I think it is usefull

Related to symfony/symfony#38021 and symfony/symfony#38021 (comment)

After my attempt at symfony/recipes#1005 and symfony/symfony#38021 (comment)

Commits
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9bc68bf Update framework.rst

Author: javiereguiluz

reference/configuration/framework.rst

@@ -37,6 +37,11 @@ used to add more entropy to security related operations. Its value should
 be a series of characters, numbers and symbols chosen randomly and the
 recommended length is around 32 characters.
 
+.. caution::
+
+    This configuration option's value is a real secret. You should treat it
+    with a lot of care.
+
 In practice, Symfony uses this value for encrypting the cookies used
 in the :doc:`remember me functionality </security/remember_me>` and for
 creating signed URIs when using :ref:`ESI (Edge Side Includes) <edge-side-includes>`.