Documentation: networking: document phy_link_topology

The newly introduced phy_link_topology tracks all ethernet PHYs that are
attached to a netdevice. Document the base principle, internal and
external APIs. As the phy_link_topology is expected to be extended, this
documentation will hold any further improvements and additions made
relative to topology handling.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>