AP_NavEKF2: documentation update

libraries/AP_NavEKF2/AP_NavEKF2_Measurements.cpp

@@ -325,8 +325,12 @@ void NavEKF2_core::readIMUData()
     // Keep track of the number of IMU frames since the last state prediction
     framesSincePredict++;
 
-    // If 10msec has elapsed, and the frontend has allowed us to start a new predict cycle, then store the accumulated IMU data
-    // to be used by the state prediction, ignoring the frontend permission if more than 20msec has lapsed
+    /*
+     * If the target EKF time step has been accumulated, and the frontend has allowed start of a new predict cycle,
+     * then store the accumulated IMU data to be used by the state prediction, ignoring the frontend permission if more
+     * than twice the target time has lapsed. Adjust the target EKF step time threshold to allow for timing jitter in the
+     * IMU data.
+     */
     if ((dtIMUavg*(float)framesSincePredict >= (EKF_TARGET_DT-(dtIMUavg*0.5)) &&
          startPredictEnabled) || (dtIMUavg*(float)framesSincePredict >= 2.0f*EKF_TARGET_DT)) {