[Sync EN] intl: document the procedural style of IntlGregorianCalendar methods

PR: `https://github.com/php/doc-en/pull/5619` (2026-08-19)

**Файлы RU для обновления**
- `reference/intl/intlgregoriancalendar/getgregorianchange.xml`
- `reference/intl/intlgregoriancalendar/isleapyear.xml`
- `reference/intl/intlgregoriancalendar/setgregorianchange.xml`

