pgsql: Doc: Fix the mistakes in the subscription's failover option.

Doc: Fix the mistakes in the subscription's failover option.

The documentation incorrectly stated that users could not alter the
subscription's failover option when the two-phase commit is enabled.

The steps to confirm that the standby server is ready for failover were
incorrect.

Author: Shveta Malik, Hou Zhijie
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/OS0PR01MB571657B72F8D75BD858DCCE394AD2@OS0PR01MB5716.jpnprd01.prod.outlook.com
Discussion: https://postgr.es/m/CAJpy0uBBk+OZXXqQ00Gai09XR+mDi2=9sMBYY0F+BedoFivaMA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/07fbecb87b16bd52b5bfe6e0ab8a1960734eb04d

Modified Files
--------------
doc/src/sgml/logical-replication.sgml    | 61 ++++++++++++++++++++------------
doc/src/sgml/ref/alter_subscription.sgml |  7 +++-
2 files changed, 44 insertions(+), 24 deletions(-)