pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/devices/signal-strength.md

@@ -2,23 +2,23 @@
 
 Returns the signal strength reported by the paired device from ``0`` (no signal) to ``4`` (full strength).
 
-### ~hint
+## ~hint
 ![](/static/bluetooth/Bluetooth_SIG.png)
 
 For another device like a smartphone to use any of the Bluetooth "services" which the @boardname@ has, it must first be [paired with the @boardname@](/reference/bluetooth/bluetooth-pairing). Once paired, the other device may connect to the @boardname@ and exchange data relating to many of the @boardname@'s features.
 
-### ~
+## ~
 
 
 ```sig
 devices.signalStrength();
 ```
 
-### Returns
+## Returns
 
 * the signal strength from ``0`` (no signal) to ``4`` (full strength).
 
-### Examples
+## Examples
 
 Display the signal strength on screen:
 
@@ -28,7 +28,7 @@ devices.onSignalStrengthChanged(() => {
 })
 ```
 
-### See Also
+## See Also
 
 [tell remote control to](/reference/devices/tell-remote-control-to), [raise alert to](/reference/devices/raise-alert-to), [on signal strength changed](/reference/devices/on-signal-strength-changed)