Document permissions needed for pg_database_size and pg_tablespace_size.
Back in 8.3, we installed permissions checks in these functions (see
commits
8bc225e7990a and
cc26599b7206). But we forgot to document that
anywhere in the user-facing docs; it did get mentioned in the 8.3 release
notes, but nobody's looking at that any more. Per gripe from Suya Huang.