net: sysfs: Document "phydev" symbolic link

Now that we link the network device to its PHY device, document this
sysfs symbolic link.
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/ABI/testing/sysfs-class-net

@@ -251,3 +251,11 @@ Contact:	netdev@vger.kernel.org
 Description:
 		Indicates the unique physical switch identifier of a switch this
 		port belongs to, as a string.
+
+What:		/sys/class/net/<iface>/phydev
+Date:		May 2017
+KernelVersion:	4.13
+Contact:	netdev@vger.kernel.org
+Description:
+		Symbolic link to the PHY device this network device is attached
+		to.