Right now, the "virtual hole" API feature of lseek(2) is very vaguely
documented and easy to miss.

At the same time, it's pretty important for anyone who is trying to use
SEEK_HOLE/SEEK_DATA in real app. Try to bridge that gap by making that
description more pronounced and also document how it affects failure codes.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D5162