pgsql: Doc: clarify the restrictions of AFTER triggers with transition

Doc: clarify the restrictions of AFTER triggers with transition tables.

It was not very clear that the triggers are only allowed on plain tables
(not foreign tables).  Also, rephrase the documentation for better
readability.

Follow up to commit 9e6104c66.

Reported-by: Etsuro Fujita <etsuro.fujita@gmail.com>
Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Etsuro Fujita <etsuro.fujita@gmail.com>
Discussion: https://postgr.es/m/CAPmGK16XBs9ptNr8Lk4f-tJZogf6y-Prz%3D8yhvJbb_4dpsc3mQ%40mail.gmail.com
Backpatch-through: 13

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5a900d6482e6e7e9d29fc51428dfd2405ded0446

Modified Files
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doc/src/sgml/ref/create_trigger.sgml | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)