Wrong comment for ReplicationSlotCreate

Hi,

I noticed that the comment for ReplicationSlotCreate function contains this
description for the "two_phase" option :

 * two_phase: Allows decoding of prepared transactions. We allow this option
 *     to be enabled only at the slot creation time. If we allow this option
 *     to be changed during decoding then it is quite possible that we skip
 *     prepare first time because this option was not enabled. Now next time
 *     during getting changes, if the two_phase option is enabled it can skip
 *     prepare because by that time start decoding point has been moved. So the
 *     user will only get commit prepared.

But commit [1] introduced the ability to alter the "two_phase" option for the
replication slot. Thus, I guess that the comment mentioned above is
outdated and we should change it.

[1] 1462aad2e4474ab61174f8ab00992cd3d6d57c7b

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Best regards,
Daniil Davydov