sys: Correct osreldate descriptions

The kern.osreldate sysctl reports the kernel version, not a release
date. Also correct a comment about /usr/include/osreldate.h.

Reviewed by: kp, olce
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50938