Documentation: net: dsa: mention simple HSR offload helpers

Keep the documentation up to date.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20251130131657.65080-16-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

Documentation/networking/dsa/dsa.rst

@@ -1119,6 +1119,14 @@ methods must be implemented:
 - ``port_hsr_leave``: function invoked when a given switch port leaves a
   DANP/DANH and returns to normal operation as a standalone port.
 
+Note that the ``NETIF_F_HW_HSR_DUP`` feature relies on transmission towards
+multiple ports, which is generally available whenever the tagging protocol uses
+the ``dsa_xmit_port_mask()`` helper function. If the helper is used, the HSR
+offload feature should also be set. The ``dsa_port_simple_hsr_join()`` and
+``dsa_port_simple_hsr_leave()`` methods can be used as generic implementations
+of ``port_hsr_join`` and ``port_hsr_leave``, if this is the only supported
+offload feature.
+
 TODO
 ====