Document permissions needed for pg_database_size and pg_tablespace_size.

Back in 8.3, we installed permissions checks in these functions (see
commits 8bc225e7 and cc26599b).  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.