pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/radio/receive-string.md

@@ -8,12 +8,12 @@ Find the next string sent by `radio` from another @boardname@.
 radio.receiveString()
 ```
 
-### Returns
+## Returns
 
 * the first [string](/types/string) that was sent. If no
   string was sent, then this function returns an empty (blank) string.
 
-### Example: Simple receiver
+## Example: Simple receiver
 
 Show the string sent by another @boardname@.
 
@@ -23,7 +23,7 @@ radio.onDataReceived(() => {
 });
 ```
 
-### Example: Two-way radio
+## Example: Two-way radio
 
 If you load this program onto two or more @boardname@s, you can send a code word from one of them to the others by pressing button `A`.
 The other @boardname@s will receive the code word and then show it.
@@ -39,13 +39,13 @@ radio.onDataReceived(() => {
 });
 ```
 
-### ~hint
+## ~hint
 
 A radio that can both transmit and receive is called a _transceiver_.
 
-### ~
+## ~
 
-### Example: Mood radio
+## Example: Mood radio
 
 This is a simple program to send whether you are happy or sad over ```radio```.
 Use the `A` or `B` button to select an emotion.
@@ -84,7 +84,7 @@ radio.onDataReceived(() => {
 });
 ```
 
-### See also
+## See also
 
 [send string](/reference/radio/send-string), [on data received](/reference/radio/on-data-received)