can: isotp: Explain PDU in CAN_ISOTP help text

The help text for the CAN_ISOTP config symbol uses the acronym "PDU".  However,
this acronym is not explained here, nor in Documentation/networking/can.rst.

Expand the acronym to make it easier for users to decide if they need to enable
the CAN_ISOTP option or not.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201013141341.28487-1-geert+renesas@glider.be
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>