usb: typec: Improve Alt Mode documentation

Adding the document to the usb index file and fixing a few
references. Also, exposing the code examples as "literal
blocks" so they are more easy to read.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>