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>