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

 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