route.8: Add information about ROUTE_MPATH and FIB_ALGO

Since the kernel options ROUTE_MPATH and FIB_ALGO are enabled
per default for a while, it's good to have some user facing
documetation about the general functionality of multipath
routing and fib lookup algorithms.

Reviewed by: pauamma, Jose Luis Duran <jlduran at gmail dot com>
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D38783