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: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>