net: phylink: clarify where phylink should be used

Update the phylink documentation to make it clear that phylink is
designed to be used on the MAC facing side of the link, rather than
between a SFP and PHY.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>