[Sync-En] ext/date: document that DateInterval addition depends on how the interval was created

[lacatoire]

Translation of php/doc-en#5830 (commit 18c0977d5e): the time part of an interval is applied as elapsed time when it comes from DateInterval::__construct, but increments component values when it comes from DateInterval::createFromDateString or DateTimeInterface::diff. Adds the new Date/Time Arithmetic example, the rule on the DateInterval class page, the note where equality was claimed, and the see-also links. EN-Revision updated.

Also picks up two older doc-en commits on dateinterval.xml (skeleton comments and empty xi:fallback removal), and fixes a missing "minutes" in the DateTimeImmutable::sub refpurpose and a reversed modality in examples.xml along the way.

Fixes: #3492