Improve two comments related to a boolean DefElem's value

Improve two comments related to a boolean DefElem's value

The original comments mentioned a "parameter" as something not defined
in a fast-exit path to assume a true status.  This is rather confusing
as the parameter DefElem is defined, and the intention is to check if
its value is defined.  This improves both comments to mention the value
assigned to the DefElem's value instead, so as future patches are able
to catch the tweak if this code pattern gets copied around more.

Author: Peter Smith
Discussion: https://postgr.es/m/CAHut+Pv0yWynWTmp4o34s0d98xVubys9fy=p0YXsZ5_sUcNnMw@mail.gmail.com