Reference to XdrvMailbox.index and example call of parser function added to section FUNC_COMMAND ID.

Sensor-API.md

@@ -135,6 +135,14 @@ This callback ID is called to allow a sensor to prepare for saving configuration
 
 ### FUNC_COMMAND
 This callback ID is called when a sensor specific command ``Sensor<xx>`` is executed where xx is the sensor index.
+```
+        case FUNC_COMMAND:
+            if (XSNS_<driver_ID> == XdrvMailbox.index) {
+               result = <driver_name>Command() { ... };  // Return true on success
+            }
+            break;
+ 
+```
 ```
 // Data struct of FUNC_COMMAND ID
 struct XDRVMAILBOX {