AP_InertialSensor: add doc to get_id() of backend

This is different from get_id() of Device and ultimately from each
sensor this this is identifying the backend, not the sensor.

libraries/AP_InertialSensor/AP_InertialSensor_Backend.h

@@ -64,6 +64,10 @@ public:
      */
     virtual AuxiliaryBus *get_auxiliary_bus() { return nullptr; }
 
+    /*
+     * Return the unique identifier for this backend: it's the same for
+     * several sensors if the backend registers more gyros/accels
+     */
     int16_t get_id() const { return _id; }
 
 protected: