improve comments on CreateFunction returnsSet

Hi hackers,

I'm using postgres, and I found a potentially ambiguous code comment in the
`CreateFunction` function. `returnsSet` is a boolean variable, and to be
consistent with the subsequent error message, I believe the comment should
be modified to match the error message. "reject ROWS if it's not
returnsSet" should be changed to "reject ROWS if it does not return a set"
 Therefore, I've attached a patch to address this issue.

Thanks!
Br/Shengbin Zhao