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.beAcked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

net/can/Kconfig

@@ -62,8 +62,9 @@ config CAN_ISOTP
 	  communication between CAN nodes via two defined CAN Identifiers.
 	  As CAN frames can only transport a small amount of data bytes
 	  (max. 8 bytes for 'classic' CAN and max. 64 bytes for CAN FD) this
-	  segmentation is needed to transport longer PDUs as needed e.g. for
-	  vehicle diagnosis (UDS, ISO 14229) or IP-over-CAN traffic.
+	  segmentation is needed to transport longer Protocol Data Units (PDU)
+	  as needed e.g. for vehicle diagnosis (UDS, ISO 14229) or IP-over-CAN
+	  traffic.
 	  This protocol driver implements data transfers according to
 	  ISO 15765-2:2016 for 'classic' CAN and CAN FD frame types.
 	  If you want to perform automotive vehicle diagnostic services (UDS),