pgsql: Doc: Improve logical replication failover documentation.

Doc: Improve logical replication failover documentation.

Clarified that the failover steps apply to a specific PostgreSQL subscriber
and added guidance for verifying replication slot synchronization during
planned failover. Additionally, corrected the standby query to avoid false
positives by checking invalidation_reason IS NULL instead of conflicting.

Author: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Author: Shveta Malik <shveta(dot)malik(at)gmail(dot)com>
Backpatch-through: 17, where it was introduced
Discussion: https://www.postgresql.org/message-id/CAExHW5uiZ-fF159=jwBwPMbjZeZDtmcTbN+hd4mrURLCg2uzJg@mail.gmail.com

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5d9e675b36b5c8ca0b2b03c2194f034b4820d4d5

Modified Files
--------------
doc/src/sgml/logical-replication.sgml | 42 ++++++++++++++++++++++++++++++-----
1 file changed, 37 insertions(+), 5 deletions(-)