Add note about unused arguments for pg_replication_origin_xact_reset() in docs.

Add note about unused arguments for pg_replication_origin_xact_reset() in docs.

In 9.5, two arguments were introduced into pg_replication_origin_xact_reset()
by mistake while they are actually not used at all. We cannot fix this issue
for 9.5 anymore because it needs a catalog version bump. Instead, we add
a note about those unused arguments into the document.

Reviewed-By: Andres Freund