pgsql: Clean-ups for 776621a5e4 and 7329240437.

Clean-ups for 776621a5e4 and 7329240437.

Following are a few clean-ups related to failover option support in slots:
1. Improve the documentation in create_subscription.sgml.
2. Remove the spurious blank line in subscriptioncmds.c.
3. Remove the NOTICE for alter_replication_slot in subscriptioncmds.c as
we would sometimes print it even when nothing has changed. One can find
the change by enabling log_replication_commands on the publisher.
4. Optimize ReplicationSlotAlter() function to prevent disk flushing when
the slot's data remains unchanged.

Author: Hou Zhijie
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com
Discussion: https://postgr.es/m/OS0PR01MB57164904651FB588A518E98894472@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/22f7e61a63306873211e26c7bac9c68631309aef

Modified Files
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doc/src/sgml/ref/create_subscription.sgml |  5 ++---
doc/src/sgml/ref/pg_dump.sgml             |  4 +---
src/backend/commands/subscriptioncmds.c   |  8 --------
src/backend/replication/slot.c            | 14 +++++++++-----
4 files changed, 12 insertions(+), 19 deletions(-)