ifconfig.8: Add capability codes descriptions from handbook

We document capability codes (the CAPS field of "ifconfig wlan0 scan")
in both ifconfig(8) and the handbook. The list is more complete in the
manual page, while the descriptions of individual capabilities are more
detailed in the handbook.

In order to reduce content duplication and bit rot, let's move
handbook's details to the manual page and reference the manual page
whenever necessary.

Reviewed by: debdrup
Reviewed by: Pau Amma
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D34662