can: j1939: add tables for the CAN identifier and its fields

Use table markup to show the structure of the CAN identifier, PGN, PDU1, and
PDU2 formats. Also add introductory sentence.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Link: https://lore.kernel.org/r/20201104155730.25196-1-yegorslists@googlemail.com
[mkl: removed trailing whitespace]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>