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: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>