doc: point out code examples by correct fomat

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
isotp: add receive timestamp to PDU as known from rawmode

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
doc: added isotp specific documentation

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>