docs: net: dsa: document the new methods for bridge TX forwarding offload

Two new methods have been introduced, add some verbiage about what they do.

Signed-off-by: Vladimir Oltean <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/dsa/dsa.rst

···
   CPU port, and flooding towards the CPU port should also be enabled, due to a
   lack of an explicit address filtering mechanism in the DSA core.
 
+- ``port_bridge_tx_fwd_offload``: bridge layer function invoked after
+  ``port_bridge_join`` when a driver sets ``ds->num_fwd_offloading_bridges`` to
+  a non-zero value. Returning success in this function activates the TX
+  forwarding offload bridge feature for this port, which enables the tagging
+  protocol driver to inject data plane packets towards the bridging domain that
+  the port is a part of. Data plane packets are subject to FDB lookup, hardware
+  learning on the CPU port, and do not override the port STP state.
+  Additionally, replication of data plane packets (multicast, flooding) is
+  handled in hardware and the bridge driver will transmit a single skb for each
+  packet that needs replication. The method is provided as a configuration
+  point for drivers that need to configure the hardware for enabling this
+  feature.
+
+- ``port_bridge_tx_fwd_unoffload``: bridge layer function invoken when a driver
+  leaves a bridge port which had the TX forwarding offload feature enabled.
+
 Bridge VLAN filtering
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