pgsql: Improve comments for [Heap]CheckForSerializableConflictOut().

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7aa4fb592530b74bf05f62c52b736ee3910691b9

Modified Files
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src/backend/access/heap/heapam.c     | 12 +++++-------
src/backend/storage/lmgr/predicate.c | 15 +++++++++------
2 files changed, 14 insertions(+), 13 deletions(-)