nl80211: better document CMD_ROAM behavior

The docs were very sparse with how exactly CMD_ROAM should be
used. Specifically related to BSS information normally obtained
through a user space scan.

Signed-off-by: James Prestwood <prestwoj@gmail.com>
Link: https://lore.kernel.org/r/20210311230333.103934-1-prestwoj@gmail.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>