[Sync EN] ext/intl: Add negative $offset for grapheme_strr functions (#4864)

Commit: `https://github.com/php/doc-en/commit/2583cd865645eea478d9a9b265d95c28a1ee5525`
PR: `https://github.com/php/doc-en/pull/4864`

**Fichiers FR à mettre à jour**
- `reference/intl/grapheme/grapheme-strripos.xml`
- `reference/intl/grapheme/grapheme-strrpos.xml`
