minor #9865 Added a small note about dynamic role hierarchies (javiereguiluz)

This PR was squashed before being merged into the 2.8 branch (closes #9865).

Discussion
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Added a small note about dynamic role hierarchies

This fixes #9745.

@dmaicher would this pull request be in line with your comments here: symfony/symfony#21853 (comment) ? Thanks!

Commits
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5230e56 Added a small note about dynamic role hierarchies

Author: javiereguiluz

security.rst

@@ -1254,6 +1254,13 @@ In the above configuration, users with ``ROLE_ADMIN`` role will also have the
 ``ROLE_USER`` role. The ``ROLE_SUPER_ADMIN`` role has ``ROLE_ADMIN``, ``ROLE_ALLOWED_TO_SWITCH``
 and ``ROLE_USER`` (inherited from ``ROLE_ADMIN``).
 
+.. note::
+
+    The value of the ``role_hierarchy`` option is defined statically, so you
+    can't for example store the role hierarchy in a database. If you need that,
+    create a custom :doc:`security voter </security/voters>` that looks for the
+    user roles in the database.
+
 Final Words
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