Adds a 'Usage from a plugin' subsection to the v9 section: build + designate
inside one push_undo scope; refresh in place via clone -> edit -> replace
(the designation travels with the entity); refusal and idempotency rules;
and the snapshot-path note that out-of-process plugins resolve entities by
iterating document().entities() (a received DocumentSnapshot does not serve
get_entity lookups).
Sebastian
06ee0798a8
