docs: net: dsa: add more info about the other arguments to get_tag_protocol

Changes were made to the prototype of get_tag_protocol without
describing at a high level what they are about. Update the documentation
to explain that.

Fixes: 5ed4e3eb ("net: dsa: Pass a port to get_tag_protocol()")
Fixes: 4d776482 ("net: dsa: Get information about stacked DSA protocol")
Signed-off-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/dsa/dsa.rst

@@ -594,7 +594,11 @@ Switch configuration
 --------------------
 
 - ``get_tag_protocol``: this is to indicate what kind of tagging protocol is
-  supported, should be a valid value from the ``dsa_tag_protocol`` enum
+  supported, should be a valid value from the ``dsa_tag_protocol`` enum.
+  The returned information does not have to be static; the driver is passed the
+  CPU port number, as well as the tagging protocol of a possibly stacked
+  upstream switch, in case there are hardware limitations in terms of supported
+  tag formats.
 
 - ``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,