[Sync EN] Document that DateInterval addition depends on how the interval was created

PR: `https://github.com/php/doc-en/pull/5830` (2026-09-03)

**Файлы RU для обновления**
- `reference/datetime/dateinterval.xml`
- `reference/datetime/dateinterval/createfromdatestring.xml`
- `reference/datetime/datetimeimmutable/add.xml`
- `reference/datetime/datetimeimmutable/sub.xml`
- `reference/datetime/examples.xml`

