minor SELECT doc patch

The documentation for SELECT is incorrect in a sense: the syntax for a
join is defined as:

from_item [ NATURAL ] join_type from_item
[ ON join_condition | USING ( join_column_list ) ]

However, if the join_type is an INNER or OUTER join, an ON, USING, or
NATURAL clause *must* be specified (it's not optional, as that segment
of the docs suggest).

I'm not exactly sure what the best way to fix this is, so I've attached
a patch adding a FIXME comment to the relevant section of the SGML. If
anyone has any ideas on the proper way to outline join syntax, please
speak up.

Cheers,

Neil
--
Neil Conway <neilc(at)samurai(dot)com> || PGP Key ID: DB3C29FC