minor #6100 [Cookbook][Security] add back updateUserSecurityIdentity() hint (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
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[Cookbook][Security] add back updateUserSecurityIdentity() hint

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.7+
| Fixed tickets | #6099

Commits
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466e404 add back updateUserSecurityIdentity() hint

Author: xabbuh

cookbook/security/acl_advanced.rst

@@ -45,6 +45,14 @@ Security Identities
 This is analog to the object identity, but represents a user, or a role in
 your application. Each role, or user has its own security identity.
 
+.. caution::
+
+    For users, the security identity is based on the username. This means that,
+    if for any reason, a user's username was to change, you must ensure its
+    security identity is updated too. The
+    :method:`MutableAclProvider::updateUserSecurityIdentity() <Symfony\\Component\\Security\\Acl\\Dbal\\MutableAclProvider::updateUserSecurityIdentity>`
+    method is there to handle the update.
+
 Database Table Structure
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