docs: net: dsa: add a section for address databases

docs: net: dsa: add a section for address databases

The given definition for what VID 0 represents in the current
port_fdb_add and port_mdb_add is blatantly wrong. Delete it and explain
the concepts surrounding DSA's understanding of FDB isolation.

Fixes: c26933639b54 ("net: dsa: request drivers to perform FDB isolation")
Signed-off-by: Vladimir Oltean <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: David S. Miller <[email protected]>