pgsql: Revisit cosmetics of "For inplace update, send nontransactional

Revisit cosmetics of "For inplace update, send nontransactional invalidations."

This removes a never-used CacheInvalidateHeapTupleInplace() parameter.
It adds README content about inplace update visibility in logical
decoding.  It rewrites other comments.

Back-patch to v18, where commit 243e9b40f1b2dd09d6e5bf91ebf6e822a2cd3704
first appeared.  Since this removes a CacheInvalidateHeapTupleInplace()
parameter, expect a v18 ".abi-compliance-history" edit to follow.  PGXN
contains no calls to that function.

Reported-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reported-by: Ilyasov Ian <ianilyasov@outlook.com>
Reviewed-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Surya Poondla <s_poondla@apple.com>
Discussion: https://postgr.es/m/CA+renyU+LGLvCqS0=fHit-N1J-2=2_mPK97AQxvcfKm+F-DxJA@mail.gmail.com
Backpatch-through: 18

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/64bf53dd61ea3224020bb340725a4df6a27bc974

Modified Files
--------------
src/backend/access/heap/README.tuplock   | 32 ++++++++++++++++++++++++++++++++
src/backend/access/heap/heapam.c         | 30 ++++++++++++++----------------
src/backend/replication/logical/decode.c | 15 +++------------
src/backend/utils/cache/inval.c          | 10 +++++++---
src/include/utils/inval.h                |  3 +--
5 files changed, 57 insertions(+), 33 deletions(-)