doc: improve `--pending-deprecation` documentation
This implements steps 1-2 from #18417 (closed):
-
--pending-deprecation
is being mentioned in COLLABORATOR_GUIDE.md#deprecations as a subset of doc-deprecation. -
--pending-deprecation
is being mentioned in doc/api/deprecations.md, and the deprecations that support it are explicitly labeled as such (currently, there is only one).
Checklist
-
make lint-md
passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc