Doc: Warn about two_phase when altering a subscription's slot name.

Doc: Warn about two_phase when altering a subscription's slot name.

We expect the 'two_phase' and 'failover' properties to match between the
slot on the publisher and a subscription option on the subscriber.
Otherwise, the slot on the publisher may behave differently from what the
subscription's failover option says.

Author: Bertrand Drouvot
Reviewed-by: Peter Smith, Tristen Raab, Amit Kapila
Discussion: https://postgr.es/m/ZbkYrLPhH+RxpZlW@ip-10-97-1-34.eu-west-3.compute.internal