pg_upgrade: Mention that we preserve database OIDs in a comment.

Oversight in commit aa01051418.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/4055696.1744134682%40sss.pgh.pa.us

src/bin/pg_upgrade/pg_upgrade.c

@@ -32,6 +32,9 @@
  *	We control all assignments of pg_authid.oid for historical reasons (the
  *	oids used to be stored in pg_largeobject_metadata, which is now copied via
  *	SQL commands), that might change at some point in the future.
+ *
+ *	We control all assignments of pg_database.oid because we want the directory
+ *	names to match between the old and new cluster.
  */