net: Extend Kernel GTP-U tunneling documentation

* clarify specification references for v0/v1
* add section "APN vs. Network device"
* add section "Local GTP-U entity and tunnel identification"

Signed-off-by: Andreas Schultz <aschultz@tpip.net>
Signed-off-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: David S. Miller <davem@davemloft.net>