machine: clarify caller's responsibility in SetInterrupt

src/machine/machine_k210.go

@@ -172,7 +172,8 @@ func (p Pin) Get() bool {
 var pinCallbacks [32]func(Pin)
 
 // SetInterrupt sets an interrupt to be executed when a particular pin changes
-// state.
+// state. The pin should already be configured as an input, including a pull up
+// or down if no external pull is provided.
 //
 // You can pass a nil func to unset the pin change interrupt. If you do so,
 // the change parameter is ignored and can be set to any value (such as 0).