Specify description of the SpecialJoinInfo structure

Hi,

Working on Asymmetric Join, I found slight inconsistency in the 
description of SpecialJoinInfo: join type JOIN_ANTI can be accompanied 
by a zero value of the ojrelid if this join was created by the 
transformation of the NOT EXISTS subquery.

-- 
regards,
Andrei Lepikhov
Postgres Professional