can: update documentation for DLC usage in Classical CAN

The extension of struct can_frame with the len8_dlc element and the
can_dlc naming issue required an update of the documentation.

Additionally introduce the term 'Classical CAN' which has been established
by CAN in Automation to separate the original CAN2.0 A/B from CAN FD.

Updated some data structures and flags.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/r/20201110101852.1973-7-socketcan@hartkopp.net


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>