pgsql: 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have become outdated.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(at)gmail(dot)com>
Author: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Backpatch-through: 18
Discussion: https://postgr.es/m/CAJDiXggZXQZ7bD0QcTizDt6us9aX6ZKK4dWxzgb5x3+TsVHjqQ@mail.gmail.com

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/aa40615cc99211db70429ebbc4c8a1549dcc06ae

Modified Files
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src/backend/replication/slot.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)