[Sync EN] Document missing file mode element for descriptors (#5079)

[github-actions]

Commit: https://github.com/php/doc-en/commit/f50098447455250c9f92eed119248aaa30920100
PR: https://github.com/php/doc-en/pull/5079

Zu aktualisierende DE-Dateien

• reference/exec/functions/proc-open.xml