docs: usb: rename files to .rst and add them to drivers-api

While there are a mix of things here, most of the stuff
were written from Kernel developer's PoV. So, add them to
the driver-api book.

A follow up for this patch would be to move documents from
there that are specific to sysadmins, adding them to the
admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/misc/Kconfig

@@ -51,7 +51,7 @@ config USB_RIO500
 	tristate "USB Diamond Rio500 support"
 	help
 	  Say Y here if you want to connect a USB Rio500 mp3 player to your
-	  computer's USB port. Please read <file:Documentation/usb/rio.txt>
+	  computer's USB port. Please read <file:Documentation/usb/rio.rst>
 	  for more information.
 
 	  To compile this driver as a module, choose M here: the