[NET]: netdevice mtu assumptions documentation

Document the expectations about device MTU handling.
The documentation about oversize packet handling is probably too
loose.

IMHO devices should drop oversize packets for robustness,
but many devices allow it now. For example, if you set mtu to 1200
bytes, most ether devices will allow a 1500 byte frame in.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>