machine: clarify caller's responsibility in SetInterrupt

src/machine/machine_nrf.go

@@ -72,7 +72,8 @@ func (p Pin) Get() bool {
 }
 
 // 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.
 //
 // This call will replace a previously set callback on this pin. You can pass a
 // nil func to unset the pin change interrupt. If you do so, the change