In commit d78bfa1ecc (Utilities/Sphinx: support cmakedomain running in
parallel, 2022-10-24, v3.26.0-rc1~495^2) we declared the domain as
parallel-safe without actually implementing the required
`merge_domaindata` method. Since then, commit 37e015d4a6
(Utilities/Sphinx: Refactor Sphinx reference recording) changed how we
store resolved references, such that our separate fix for 3.26 does not
work in 3.27+.
While at it, correct a crusty comment that was overlooked during the
aforementioned refactoring.
Issue: #24076
Co-authored-by: Jared Dillard <jared.dillard@gmail.com>