minor #9552 [Security] add tip about using custom functions (dmaicher, javiereguiluz)

This PR was merged into the master branch.

Discussion
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[Security] add tip about using custom functions

Since 4.1 its possible to use custom functions inside security `allow_if` expressions.

See symfony/symfony#26660

Commits
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44708ad Reword
8a96822 [Security] add tip about using custom functions

Author: javiereguiluz

security/access_control.rst

@@ -301,6 +301,15 @@ and functions including ``request``, which is the Symfony
 For a list of the other functions and variables, see
 :ref:`functions and variables <security-expression-variables>`.
 
+.. tip::
+
+    The ``allow_if`` expressions can also contain custom functions registered
+    with :ref:`expression providers <components-expression-language-provider>`.
+
+    .. versionadded:: 4.1
+        The feature to use custom functions inside ``allow_if`` expressions was
+        introduced in Symfony 4.1.
+
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