Documentation: net: dsa: mention availability of RedBox

Since commit 5055cccfc2d1 ("net: hsr: Provide RedBox support
(HSR-SAN)"), RedBox is available (including for offload in DSA).

Update the DSA documentation that states it isn't.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20251130131657.65080-15-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>