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

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bae8ca82fd00603ebafa0658640d6e4dfe20af92

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(-)