Clarify ultrasound

Author: WillB97

programming/arduino/sr_firmware.md

@@ -133,6 +133,8 @@ This means you can simply connect a switch between the input pin and a ground pi
 
 You can also measure distance using an ultrasound sensor from the arduino. Ultrasound sensors return the distance of the closest object in mm.
 
+To use this on your robot you will need use HC-SR04 ultrasound modules.
+
 ```python
 # Trigger pin: 4
 # Echo pin: 5
@@ -142,6 +144,7 @@ distance_mm = robot.arduino.ultrasound_measure(4, 5)
 ```
 
 <div class="warning">
-The ultrasound sensor can measure distances up to 4 metres.
-If the ultrasound signal has to travel further than 4m, the sensor will timeout and return 0.
+The ultrasound sensor can measure distances up to around 4 metres.
+If the ultrasound signal has to travel further, the echo may not be detected. 
+This will cause the sensor to timeout and return 0.
 </div>