ifnet(9): Document the IFF_PALLMULTI flag
It was introduced by the recent commit [1].
- 58f194223ab8 ifnet: Add handling for toggling IFF_ALLMULTI in ifhwioctl()
Reviewed by: markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D47511
(cherry picked from commit f9f470f72c8718c3503874791d593997c3d751b4)