cdboot.8: Initial manual page

Add cdboot to the reference manual, fixing an undocumented bug where it
is undocumented. There's almost nothing here, but that's better than
"only imp and jhb know what this is".

MFC after: 3 days
Reported by: jhb, imp
Reviewed by: mhorne
Approved by: mhorne (mentor)
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D50274

(cherry picked from commit ecdb0b79bf93a2b8392694bd266c5930ca42bab5)
(cherry picked from commit d659366cc62a70b4863bddf226f6209d13ab65c0)