USB: power-management documenation update

This patch (as1014) was partly written by Tilman Schmidt.  It
clarifies the USB power-management documentation by explaining that
when a disconnect occurs, a suspend method call might not be followed
by either a resume or a reset_resume call.
Signed-off-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NTilman Schmidt <tilman@imap.cc>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/usb/power-management.txt

@@ -278,6 +278,14 @@ optional.  The methods' jobs are quite simple:
 	(although the interfaces will be in the same altsettings as
 	before the suspend).
 
+If the device is disconnected or powered down while it is suspended,
+the disconnect method will be called instead of the resume or
+reset_resume method.  This is also quite likely to happen when
+waking up from hibernation, as many systems do not maintain suspend
+current to the USB host controllers during hibernation.  (It's
+possible to work around the hibernation-forces-disconnect problem by
+using the USB Persist facility.)
+
 The reset_resume method is used by the USB Persist facility (see
 Documentation/usb/persist.txt) and it can also be used under certain
 circumstances when CONFIG_USB_PERSIST is not enabled.  Currently, if a