Documentation: net: phy: Add a paragraph about pause frames/flow control

Describe that the Ethernet MAC controller is ultimately responsible for
dealing with proper pause frames/flow control advertisement and
enabling, and that it is therefore allowed to have it change
phydev->supported/advertising with SUPPORTED_Pause and
SUPPORTED_AsymPause.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>