Updated Sensor API (markdown)

Sensor-API.md

@@ -102,7 +102,28 @@ This callback ID is called every 50 milliseconds, e.g. for near real-time operat
 This callback ID is called every second.
 
 ### FUNC_PREP_BEFORE_TELEPERIOD
-???
+This callback ID is called before TELEPERIOD and is intended.
+
+You would normally want to make sure you've detected and initialised before it is used by JSON_APPEND etc so that its ready to serve data.
+
+The generally accepted way to use this would be to detect your sensor and once this is done set a sensor value accordingly so that the function does not use unnecessary resources during future calls, for example:
+```
+void MySensorDetect()
+{
+  if (MySensorDetected) { return; }
+  /*
+   * Perform the code which needs to be completed to
+   * detect your sensor and then set MySensorDetected to
+   * a non-zero value which will prevent this section
+   * of your code to re-run every time the function is
+   * called.
+   * 
+   * Under normal circumstances you'd not need to do
+   * re-detect or initialise your sensor once it has been
+   * done
+   */
+}
+```
 
 ### FUNC_JSON_APPEND
 This callback ID is called when `TELEPERIOD` is due to append telemetry data to the MQTT JSON string, e.g.