[media] doc-rst: add documentation for pvrusb2

Convert pvrusb2 documentation to ReST file and removed the note
about an html version of the documentation, as it is not
shipped inside the Kernel.

Add it to media/v4l-drivers book.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/v4l-drivers/index.rst

@@ -30,4 +30,5 @@ License".
 	meye
 	omap3isp
 	omap4_camera
+	pvrusb2
 	zr364xx