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