pgsql: 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: <duffieldzane(at)gmail(dot)com>
Author: Hou Zhijie <houzj(dot)fnst(at)fujitsu(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Backpatch-through: 17, where it was introduced
Discussion: https://postgr.es/m/173976850802.682632.11315364077431550250@wrigleys.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5b8f2ccc0a93375acb64a457817e61f400404a1f

Modified Files
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doc/src/sgml/func.sgml | 4 ++++
src/backend/replication/slotfuncs.c | 19 ++++++++++++-------
2 files changed, 16 insertions(+), 7 deletions(-)