docs: net: dsa: sja1105: update list of limitations

Remove the paragraphs that talk about the various modes of traffic
support, bridging with foreign interfaces, etc etc. There is nothing
that the user needs to know now, it should all work out of the box as
expected.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>