uname(1): Clarify the -r option

[gbe]

Summary

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
MFC after: 1 week

Test Plan

mandoc output review and 'mandoc -Tlint' checks

[imp]

usr.bin/uname/uname.1

85–89

Why not just s/operating system/the kernel/ in the prior sentence and not insert the first of these two sentences?

[gbe]

• Condense the description of the -r flag