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 
Author: shveta malik 
Reviewed-by: Zhijie Hou 
Reviewed-by: Peter Smith 
Reviewed-by: Amit Kapila 
Backpatch-through: 17, where it was introduced
Discussion: https://postgr.es/m/CAF1DzPWTcg+m+x+oVVB=y4q9=PYYsL_mujVp7uJr-_oUtWNGbA@mail.gmail.com