uname.1: Clarify the -r option

It is some times hard to understand the difference between
kernel version and userland version. So clarify the -r option
of uname(1) in terms of a printed kernel version.

While here, add some cross references:

• cross reference freebsd-version(1) in uname(1)
• cross reference freebsd-version(1) and uname(1) in freebsd-update(8)

PR: 265594
Reported by: rwatson
Reviewed by: gbe, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36516