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

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