Update comments atop ReplicationSlotCreate.

Update comments atop ReplicationSlotCreate.

Since commit 1462aad2e4, which introduced the ability to modify the
two_phase property of a slot, the comments above ReplicationSlotCreate
have become outdated. We have now added a cautionary note in the comments
above ReplicationSlotAlter explaining when it is safe to modify the
two_phase property of a slot.

Author: Daniil Davydov <3danissimo@gmail.com>
Author: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Backpatch-through: 18
Discussion: https://postgr.es/m/CAJDiXggZXQZ7bD0QcTizDt6us9aX6ZKK4dWxzgb5x3+TsVHjqQ@mail.gmail.com