Doc: Fix pg_copy_logical_replication_slot description.

Doc: Fix pg_copy_logical_replication_slot description.

This commit documents that the failover option is not copied when using
the pg_copy_logical_replication_slot function.

In passing, we modify the comments in the function clarifying the reason
for this behavior.

Reported-by: <[email protected]>
Author: Hou Zhijie <[email protected]>
Reviewed-by: Amit Kapila <[email protected]>
Backpatch-through: 17, where it was introduced
Discussion: https://postgr.es/m/173976850802.682632.11315364077431550250@wrigleys.postgresql.org