usb: documentation for usb port power off mechanisms

describe the mechanisms for controlling port power policy and
discovering the port power state.

[oliver]: fixes, clarification of wakeup vs port-power-control
[sarah]: wordsmithing
[djbw]: updates for peer port changes
[alan]: review and fixes
Cc: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>