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

[github-actions]

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

Neue EN-Dateien (noch nicht übersetzt)

• reference/intl/grapheme/grapheme-strripos.xml
• reference/intl/grapheme/grapheme-strrpos.xml