clarifying trigger/rule behavior on logical replication subscribers

Hi,

While answering a question on "do triggers fire on a logical replication 
subscriber by default?" I tried to look up a reference to this behavior 
in the docs. There wasn't a clear reference point, but on the 
architecture page[1], I found this line that was closest to the answer:

"The apply process on the subscriber database always runs with 
session_replication_role set to replica, which produces the usual 
effects on triggers and constraints."

which assumes that the reader knows what the "usual effects" are.

Attached is a patch that disambiguates this.

Two additional items:

1. I do think this should be backpatched because it clarifies an 
existing behavior.

2. I don't know if we want to add a note about the trigger/rule behavior 
elsewhere in the logical replication section. I don't know if a reader 
would be expecting to find this in the "architecture" section.

Thanks,

Jonathan

[1] 
https://www.postgresql.org/docs/devel/logical-replication-architecture.html