Improve slightly misleading comments in nodeFuncs.c

Improve slightly misleading comments in nodeFuncs.c

There were some comments in nodeFuncs.c that, depending on your
interpretation of the word "result", could lead you to believe that the
comments were badly copied and pasted from somewhere else.  If you thought
of "result" as the return value of the function that the comment is
written in, then you'd be misled.  However, if you'd correctly
interpreted "result" to mean the result type of the given node type,
you'd not have seen any issues.

Here we do a small cleanup to try to prevent any future
misinterpretations.  Per wording suggestion from Tom Lane.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAApHDvp+Bw=2Qiu5=uXMKfC7gd0+B=4JvexVgGJU=am2g9a1CA@mail.gmail.com