usb: documentation for usb port power off mechanisms

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

Cc: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
[sarah]: wordsmithing
[djbw]: updates for peer port changes
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
1 file changed