Update Documentation/driver-api/usb/usb3-debug-port.rst. This update
includes the guide for using xHCI debug capability based TTY serial
link.

Cc: Mathias Nyman <mathias.nyman@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NLu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>