[Sync EN] unserialization: Documented allowed-classes option with enums (#5585)

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Commit: https://github.com/php/doc-en/commit/8af3521cb43f54c652baaf8dbbcb786b79a5d04e
PR: https://github.com/php/doc-en/pull/5585

Zu aktualisierende DE-Dateien

• language/enumerations.xml
• reference/var/functions/unserialize.xml