Fix permissions explanations in CREATE DATABASE and CREATE SCHEMA docs.

These reference pages still claimed that you have to be superuser to create
a database or schema owned by a different role.  That was true before 8.1,
but it was changed in commits aa111062 and
f91370cd to allow assignment of ownership
to any role you are a member of.  However, at the time we were thinking of
that primarily as a change to the ALTER OWNER rules, so the need to touch
these two CREATE ref pages got missed.