docs: net: dsa: document change_tag_protocol

Support for changing the tagging protocol was added without this
operation being documented; do so now.

Fixes: 53da0eba ("net: dsa: allow changing the tag protocol via the "tagging" device attribute")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/dsa/dsa.rst

@@ -600,6 +600,12 @@ Switch configuration
   upstream switch, in case there are hardware limitations in terms of supported
   tag formats.
 
+- ``change_tag_protocol``: when the default tagging protocol has compatibility
+  problems with the master or other issues, the driver may support changing it
+  at runtime, either through a device tree property or through sysfs. In that
+  case, further calls to ``get_tag_protocol`` should report the protocol in
+  current use.
+
 - ``setup``: setup function for the switch, this function is responsible for setting
   up the ``dsa_switch_ops`` private structure with all it needs: register maps,
   interrupts, mutexes, locks, etc. This function is also expected to properly