documenting forbidden expressions in column defaults

Hi.

The description of DEFAULT in the documentation currently says:

"The value is any variable-free expression (subqueries and
cross-references to other columns in the current table are not allowed)."

But the value cannot be an aggregate expression, a window function, or a
set-returning function either, so maybe we should list these items too?

Attached is a patch that fixes the text in two places where the above
description appears.

Thanks,
Amit