For the new buildkernel/installkernel targets, /usr/src/* must exist.
release-versions of FreeBSD might only contain /usr/src/sys.
The old description lead to misunderstandings for newbies.
Clarify that you still have to use the old method if you don't sync src/
with the FreeBSD project.

PR: 20136
Reported by: Stephen Montgomery-Smith <stephen@math.missouri.edu>

		(different patch suggested)