ar40xx.4: Initial manual page

For inclusion in apropos and the hardware release notes. While here,
add xrefs to this page and the previous e6000sw in etherswitch.4.

MFC after: 3 days (but adjusted for 14.3)
Reported by: adrian
Reviewed by: adrian, mhorne
Approved by: mhorne (mentor)
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D50324

(cherry picked from commit 91c9751c3913497db2fbad1fcf7491557bbd2a75)
(cherry picked from commit 17e9eb1e0eb721e2400232c71f20bc490c416c60)