usb/URB.txt: convert to ReST and update it

The URB doc describes the Kernel mechanism that do USB transfers.
While the functions are already described at urb.h, there are a
number of concepts and theory that are important for USB driver
developers.

Convert it to ReST and use C ref links to point to the places
at usb.h where each function and struct is located.

A few of those descriptions were incomplete. While here, update
to reflect the current API status.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>