The previous comment gave the impression that skipping OIDs before
FirstNormalObjectId was merely an optimization to avoid likely collisions.
In fact other parts of the system have been relying on this threshold to
detect system-created objects since commit 8e18d04d, so adjust the
wording.

Author: Thomas Munro
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAEepm%3D33JASACeOayr_W3%3DCSjy2jiPxM-k89axu0akFbHdjnjA%40mail.gmail.com