minor #16700 Fix invalid interface mention in security.rst (ttibensky)

This PR was merged into the 5.4 branch.

Discussion
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Fix invalid interface mention in security.rst

SerializableInterface doesn't exist, I assume it was meant to be https://www.php.net/manual/en/class.serializable.php, the motivation behind this change is described here https://stackoverflow.com/questions/42074225/symfony-userinterface-is-serializing-the-entire-massive-user-entity/71816482#71816482

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Commits
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6e7b827 No longer mention a deprecated interface
c1993e8 Fix invalid interface mention in security.rst

Author: wouterj

security.rst

@@ -2503,9 +2503,10 @@ However, in some cases, this process can cause unexpected authentication problem
 If you're having problems authenticating, it could be that you *are* authenticating
 successfully, but you immediately lose authentication after the first redirect.
 
-In that case, review the serialization logic (e.g. ``SerializableInterface``) on
-you user class (if you have any) to make sure that all the fields necessary are
-serialized.
+In that case, review the serialization logic (e.g. the ``__serialize()`` or
+``serialize()`` methods) on you user class (if you have any) to make sure
+that all the fields necessary are serialized and also exclude all the
+fields not necessary to be serialized (e.g. Doctrine relations).
 
 Comparing Users Manually with EquatableInterface
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~