Re: a raft of parallelism-related bug fixes

It's good to have your perspective on how this can be improved, and
I'm definitely willing to write more documentation.  Any lack in that
area is probably due to being too close to the subject area, having
spent several years on parallelism in general, and 200+ emails on
parallel sequential scan in particular.  Your point about the lack of
a good header file comment for execParallel.c is a good one, and I'll
rectify that next week.