Documentation: networking: update the graphical representation

While preparing some slides for a customer presentation, I found the
existing high-level view to be a bit confusing, so I modified it a
little bit.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>