netlink: Fix "version introduced" documentation

netlink(4) and associated features will exist in FreeBSD 14.0 but they
will also exist in 13.2, an older version, from commits such as 02b958b1
and b309249b.

This commit needs merging to stable/13 and releng/13.2.

MFC after: 2days (needs to be in RC2)
Reviewed by: imp,melifaro
Pull Request: https://github.com/freebsd/freebsd-src/pull/651
Approved by: re@ (cperciva)

(cherry picked from commit 6d2feb39ad13768f3ea06381fcd95937a09f0d62)