switchdev: documentation: use switchdev_port_obj_xxx for IPv4 FIB add/modify/delete ops

Clarify in documentation and code that IPV4 FIB add operation is used for
both adding a new FIB entry to the device and for modifying an existing FIB
entry on the device.

Also, remove left-over references to ipv4_fib ops and replace with details
on SWITCHDEV_PORT_IPV4_FIB object.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>