pgsql: Doc: clarify the conditions of usable indexes for REPLICA IDENTI

Doc: clarify the conditions of usable indexes for REPLICA IDENTITY FULL tables.

Commit 89e46da5e allowed REPLICA IDENTITY FULL tables to use an index
on the subscriber during apply of update/delete. This commit clarifies
in the documentation that the leftmost field of candidate indexes must
be a column (not an expression) that references the published relation
column.

The source code comments are also updated accordingly.

Reviewed-by: Peter Smith, Amit Kapila
Discussion: https://postgr.es/m/CAD21AoDJjffEvUFKXT27Q5U8-UU9JHv4rrJ9Ke8Zkc5UPWHLvA@mail.gmail.com
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4946910a875ce84823236f2a06e576b16080465f

Modified Files
--------------
doc/src/sgml/logical-replication.sgml      | 12 ++++++------
src/backend/executor/execReplication.c     |  6 +++---
src/backend/replication/logical/relation.c | 11 ++++-------
3 files changed, 13 insertions(+), 16 deletions(-)