route.8: Describe weight modifier

Describe `-weight` argument in the route manual.

Reviewed by: glebius, ziaee
Differential Revision: https://reviews.freebsd.org/D56246

sbin/route/route.8

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd November 12, 2024
+.Dd May 16, 2026
 .Dt ROUTE 8
 .Os
 .Sh NAME
@@ -297,6 +297,17 @@ Note that the qualifier works only for
 .Dv AF_INET6
 address family.
 .Pp
+For
+.Li ECMP
+routes, the
+.Fl weight
+modifier can be used to influence nexthop selection.
+For example, a destination with two nexthops defined as
+.Dq Fl gateway Ar 3fff::1 Fl weight Ar 100
+and
+.Dq Fl gateway Ar 3fff::2 Fl weight Ar 200
+will make the nexthop 3fff::2 twice as likely to be chosen as 3fff::1.
+.Pp
 Routes have associated flags which influence operation of the protocols
 when sending to destinations matched by the routes.
 These flags may be set (or sometimes cleared)