default parameters

According to http://www.postgresql.org/docs/current/interactive/plpgsql-porting.html
there are no default parameters in PostgreSQL, but according to
http://www.postgresql.org/docs/current/interactive/sql-createfunction.html
there are.

It looks like this was fixed in CVS HEAD at some point (maybe when the
changes regarding the scoping of column names vs. parameters were put
in) but I think we should back-patch the part that also applies to 8.4
so that the docs are internally consistent.

...Robert