Improve comment in GetNewObjectId().

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

src/backend/access/transam/varsup.c

@@ -476,11 +476,9 @@ GetNewObjectId(void)
 
 	/*
 	 * Check for wraparound of the OID counter.  We *must* not return 0
-	 * (InvalidOid); and as long as we have to check that, it seems a good
-	 * idea to skip over everything below FirstNormalObjectId too. (This
-	 * basically just avoids lots of collisions with bootstrap-assigned OIDs
-	 * right after a wrap occurs, so as to avoid a possibly large number of
-	 * iterations in GetNewOid.)  Note we are relying on unsigned comparison.
+	 * (InvalidOid), and in normal operation we mustn't return anything below
+	 * FirstNormalObjectId since that range is reserved for initdb (see
+	 * IsCatalogClass()).  Note we are relying on unsigned comparison.
 	 *
 	 * During initdb, we start the OID generator at FirstBootstrapObjectId, so
 	 * we only wrap if before that point when in bootstrap or standalone mode.