Improve comments for [Heap]CheckForSerializableConflictOut().
Rewrite the documentation of these functions, in light of recent bug fix
commit
5940ffb2.
Back-patch to 13 where the check-for-conflict-out code was split up into
AM-specific and generic parts, and new documentation was added that now
looked wrong.
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/
db7b729d-0226-d162-a126-
8a8ab2dc4443%40jepsen.io