lagg.4: update wireless example, document laggtype

The wireless example in lagg(4)'s manpage no longer works, so update.

laggtype is not documented in the manpage other than in the example,
so add a paragraph about it.

While here, move section on rc.conf somewhere it sits more logically.

Reviewed by: pauamma
Differential Revision: https://reviews.freebsd.org/D31315