nl80211: remove desciption about request from NL80211_CMD_ROAM

The description of NL80211_CMD_ROAM indicated possibility for a
request to roam issued by user-space. However, it also states that
as not being implemented right now. This has been so since commit
b23aa676 ("cfg80211: connect/disconnect API") added in 2009.
So it seems safe to assume it will not be added any time soon and
thus remove it.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>