e6000sw.4: Initial manual page

For inclusion in the HW relnotes.

MFC after: 3 days, note removed entries which didn't mfc
Reviewed by: adrian, ivy, mhorne
Approved by: mhorne (mentor)
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D50293

(cherry picked from commit 6da793a8caa15ee2cbb984ea39ac87c6cd2fdf63)
(cherry picked from commit d0e29f92f7a252b382d76c1e6bac33d8424af5fa)