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

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