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

[lacatoire]

Translation of php/doc-en#5830 (commit 18c0977): the time part of an interval is
applied differently depending on whether it was built with the constructor or
parsed from a date string, which only shows when a timezone transition falls
inside it. Adds the new arithmetic example and the note on createFromDateString.
EN-Revision updated.

Along the way, in the same files: removed the empty xi:fallback elements that
doc-en dropped in #5699, unwrapped the examples and the parameter variablelist
from their para as upstream did, and replaced one use of the informal "debes" in
createfromdatestring.xml, which GUIDE.md forbids.

Fixes: #1251