[Sync EN] apache2.xml Add notes about FallbackResource (#4580)

[github-actions]

Commit: https://github.com/php/doc-en/commit/a38e360ae1ce4169b331013958c519d17f19114e
PR: https://github.com/php/doc-en/pull/4580

Fichiers FR à mettre à jour

• install/unix/apache2.xml