netstat(1): add description for option -o and -O.

Reviewed by: kevans, ziaee, ivy
Approved by: kevans (mentor)
Fixes: fedeb08b6a58 ("Introduce scalable route multipath.")
Fixes: a666325282ea ("Introduce nexthop objects and new routing KPI.")
Pull Request: https://github.com/freebsd/freebsd-src/pull/1643

(cherry picked from commit afa30c5dec5afc8496172a062c93e96d78f5b9b9)