pgsql: Improve log messages and docs for slot synchronization.

Improve log messages and docs for slot synchronization.

Improve the clarity of LOG messages when a failover logical slot
synchronization fails, making the reasons more explicit for easier
debugging.

Update the documentation to outline scenarios where slot synchronization
can fail, especially during the initial sync, and emphasize that
pg_sync_replication_slot() is primarily intended for testing and
debugging purposes.

We also discussed improving the functionality of
pg_sync_replication_slot() so that it can be used reliably, but we would
take up that work for next version after some more discussion and review.

Reported-by: Suraj Kharage <suraj.kharage@enterprisedb.com>
Author: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 17, where it was introduced
Discussion: https://postgr.es/m/CAF1DzPWTcg+m+x+oVVB=y4q9=PYYsL_mujVp7uJr-_oUtWNGbA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1546e17f9d067e714e066fcdd57d5f56c14f4174

Modified Files
--------------
doc/src/sgml/func.sgml                     |  6 ++--
doc/src/sgml/logicaldecoding.sgml          | 54 +++++++++++++++++++++++++++---
src/backend/replication/logical/slotsync.c |  6 ++--
3 files changed, 57 insertions(+), 9 deletions(-)