Documentation: networking: switchdev: fix command for static FDB entries

The "bridge fdb add" command provided in the switchdev documentation is
junk now, not only because it is syntactically incorrect and rejected by
the iproute2 bridge program, but also because it was not updated in
light of Arkadi Sharshevsky's radical switchdev refactoring in commit
29ab586c ("net: switchdev: Remove bridge bypass support from
switchdev"). Try to explain what the intended usage pattern is with the
new kernel implementation.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>