More pg_upgrade clarifications

The attached two patches clarify pg_upgrades behavior. Specifically,
the first patch updates the C comments regarding pg_upgrade_support.

The second patch clarifies that pg_class.oid really needs preserving,
not pg_class.relfilenode. We used to use create relfilenode files to
preserve pg_class.oids, but no longer in PG >= 9.0. Also add comments
about what is being preserved, and why. This should help future
improvements.

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +