doc: Fix mistakes in the most recent set of release notes.

Improve description of the vacuum_freeze_table_age bug (it's much more
serious than we realized at the time the fix was committed), and correct
attribution of pg_upgrade -O/-o fix (Marti Raudsepp contributed that,
but Bruce forgot to credit him in the commit log).

No need to back-patch right now, it'll happen when the next set of
release notes are prepared.