Documentation: LEDs: Describe good names for network LEDs

Network LEDs can exist in both the MAC and the PHY. Naming is
difficult because the netdev name is neither stable or unique, do to
commands like ip link set name eth42 dev eth0, and network
namesspaces.

Give some example names where the MAC and the PHY have unique names
based on device tree nodes, or PCI bus addresses.

Since the LED can be used for anything which Linux supports for LEDs,
avoid using names like activity or link, rather describe the location
on the RJ-45, of what the RJ-45 is expected to be used for, WAN/LAN
etc.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>