net: phylink: clarify flow control settings in documentation

Clarify the expected flow control settings operation in the phylink
documentation for each negotiation mode.

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>