[media] rc5x: document that this is the 20 bit variant

There are many variants of extended rc5. This implements the 20 bit
version.

Signed-off-by: Sean Young <sean@mess.org>
Cc: David Härdeman <david@hardeman.nu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>