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>

Documentation/driver-api/usb/index.rst

@@ -10,6 +10,7 @@ Linux USB API
    bulk-streams
    callbacks
    dma
+   URB
    power-management
    hotplug
    persist

Documentation/driver-api/usb/usb.rst

+.. _usb-hostside-api:
+
 ===========================
 The Linux-USB Host Side API
 ===========================