In editing 09b12d52 I made it wrong;
fix that and try to more clearly explain the situation.

Patch by me, reviewed by David Rowley and Amit Langote

Discussion: http://postgr.es/m/CA+TgmobAq+mA5hzm0a5OS38qQY5758DDDGqa3sBJN4hvir-H9w@mail.gmail.com