can: update MAINTAINERS and Documentation

Changed MAINTAINERS file to add Documentation/networking/can.txt to the list of
maintained files.

can.txt:
- Globally changed Socket CAN to SocketCAN
- Removed section 3.3 from the document
- Updated Section 7
- Corrected a few simple typos

Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>