UNION DISTINCT in doc

I found PostgreSQL accepts UNION DISTINCT but documents don't mention it.

http://www.postgresql.org/docs/9.0/static/sql-select.html#SQL-UNION

select_statement UNION [ ALL ] select_statement

UNION DISTINCT is nothing more than UNION itself, but gram.y
definitely accept it and the SQL standard describes it as well. Should
we add DISTINCT to docs?

Regards,

-- 
Hitoshi Harada