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

doc/src/sgml/ref/create_trigger.sgml

@@ -197,9 +197,11 @@ CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGER <replaceable class="parameter">name
    of the rows inserted, deleted, or modified by the current SQL statement.
    This feature lets the trigger see a global view of what the statement did,
    not just one row at a time.  This option is only allowed for
-   an <literal>AFTER</literal> trigger that is not a constraint trigger; also, if
-   the trigger is an <literal>UPDATE</literal> trigger, it must not specify
-   a <replaceable class="parameter">column_name</replaceable> list.
+   an <literal>AFTER</literal> trigger on a plain table (not a foreign table).
+   The trigger should not be a constraint trigger.  Also, if the trigger is
+   an <literal>UPDATE</literal> trigger, it must not specify
+   a <replaceable class="parameter">column_name</replaceable> list when using
+   this option.
    <literal>OLD TABLE</literal> may only be specified once, and only for a trigger
    that can fire on <literal>UPDATE</literal> or <literal>DELETE</literal>; it creates a
    transition relation containing the <firstterm>before-images</firstterm> of all rows