net: phylink: expand on .pcs_config() method documentation

Expand on the requirements of the .pcs_config() method documentation,
specifically mentioning that it should cause minimal disruption to
an established link, and that it should return a positive non-zero
value when requiring the .pcs_an_restart() method to be called.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1trb24-005oVq-Is@rmk-PC.armlinux.org.uk


Signed-off-by: Paolo Abeni <pabeni@redhat.com>