Improve comments for parallel executor estimation functions.

Improve comments for parallel executor estimation functions.

The previous comment (which was copied as boilerplate from one file
to the next) implied that it was the executor node itself which was
being serialized, but that's not right.  We're not serializing
the executor nodes; we're just allowing them to store some
additional information in DSM.  Adjusts the comment to reflect this.

Discussion: http://postgr.es/m/CA+TgmoaHVinxG=3h6qBAsyV8xaDyQwbzK7YZnYfE8nJFMK1=FA@mail.gmail.com