[Sync EN] Add warning for header_register_callback usage (#5760)

[github-actions]

Commit: https://github.com/php/doc-en/commit/abdc23836d3f42399028c1174961107e70ada52d
PR: https://github.com/php/doc-en/pull/5760

Archivos ES a actualizar

• reference/network/functions/header-register-callback.xml