[Sync EN] Document that DateInterval addition depends on how the interval was created (#5830)

[github-actions]

Commit: https://github.com/php/doc-en/commit/18c0977d5ec59f4207cb0b306c5473508bf640ed
PR: https://github.com/php/doc-en/pull/5830

Fichiers FR à mettre à jour

• reference/datetime/dateinterval.xml
• reference/datetime/dateinterval/createfromdatestring.xml
• reference/datetime/datetimeimmutable/add.xml
• reference/datetime/datetimeimmutable/sub.xml
• reference/datetime/examples.xml