Add send-string documentation page

docs/reference/radio/send-string.md

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+# Send String
+
+Broadcasts a string data packet to other micro:bits connected via ``radio``.
+
+## Important Security Consideration
+
+The functions in the ``radio`` namespace allow the BBC micro:bit to communicate with other micro:bits.
+
+This API does not contain any form of encryption, authentication or authorization. It's purpose is solely for use as a teaching aid to demonstrate how simple communications operates, and to provide a sandpit through which learning can take place.
+
+For serious applications, BLE should be considered a substantially more secure alternative.
+
+```sig
+radio.sendString("Hello world!")
+```
+
+### Parameters
+
+* msg - a string to be transmitted.
+
+### Examples
+
+Broadcasts the provided string to other micro:bits.
+
+```blocks
+input.onButtonPressed(Button.A, () => {
+    radio.sendString("Mr. Watson, come here, I want to see you.")
+})
+```
+
+### See also
+
+[receive string](/reference/radio/receive-string), [on data received](/reference/radio/on-data-received)