docs: net: dsa: document the teardown method

A teardown method was added to dsa_switch_ops without being documented.
Do so now.

Fixes: 5e3f847a ("net: dsa: Add teardown callback for drivers")
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

@@ -617,7 +617,8 @@ Switch configuration
   fully configured and ready to serve any kind of request. It is recommended
   to issue a software reset of the switch during this setup function in order to
   avoid relying on what a previous software agent such as a bootloader/firmware
-  may have previously configured.
+  may have previously configured. The method responsible for undoing any
+  applicable allocations or operations done here is ``teardown``.
 
 PHY devices and link management
 -------------------------------