uwb: doc: bring uwb documentation up to date

The existing UWB documentation states that UWB devices show up in
/sys/class/uwb and /sys/bus/uwb/devices.  Up until the most recent
changes to add uwb devices to their own bus, neither of these statements
were actually true.  Now, UWB devices do show up in /sys/bus/uwb/devices
so the documentation has been updated to reflect that and removed
references to /sys/class/uwb.

The existing documentation also states that local UWB radio controller
(RC) devices also show up as UWB devices along with their wireless peer
UWB devices under /sys/class/uwb.  This is not true so that statement
has been removed.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>