Doc: Clarify the behavior of triggers/rules in a logical subscriber.

By default, triggers and rules do not fire on a logical replication
subscriber based on the "session_replication_role" GUC being set to
"replica". However, the docs in the logical replication section assumed
that the reader understood how this GUC worked. This modifies the docs to
be more explicit and links back to the GUC itself.

Author: Jonathan Katz, Peter Smith
Reviewed-by: Vignesh C, Euler Taveira
Backpatch-through: 11
Discussion: https://postgr.es/m/5bb2c9a2-499f-e1a2-6e33-5ce96b35cc4a@postgresql.org

doc/src/sgml/logical-replication.sgml

@@ -453,9 +453,13 @@
 
   <para>
    The apply process on the subscriber database always runs with
-   <varname>session_replication_role</varname> set
-   to <literal>replica</literal>, which produces the usual effects on triggers
-   and constraints.
+   <link linkend="guc-session-replication-role"><varname>session_replication_role</varname></link>
+   set to <literal>replica</literal>. This means that, by default,
+   triggers and rules will not fire on a subscriber. Users can optionally choose to
+   enable triggers and rules on a table using the
+   <link linkend="sql-altertable"><command>ALTER TABLE</command></link> command
+   and the <literal>ENABLE TRIGGER</literal> and <literal>ENABLE RULE</literal>
+   clauses.
   </para>
 
   <para>