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

include/uapi/linux/nl80211.h

@@ -655,6 +655,9 @@
  *	When a security association was established on an 802.1X network using
  *	fast transition, this event should be followed by an
  *	%NL80211_CMD_PORT_AUTHORIZED event.
+ *	Following a %NL80211_CMD_ROAM event userspace can issue
+ *	%NL80211_CMD_GET_SCAN in order to obtain the scan information for the
+ *	new BSS the card/driver roamed to.
  * @NL80211_CMD_DISCONNECT: drop a given connection; also used to notify
  *	userspace that a connection was dropped by the AP or due to other
  *	reasons, for this the %NL80211_ATTR_DISCONNECTED_BY_AP and