AP_Camera: updated docs for timing of trigger pin

libraries/AP_Camera/AP_Camera.cpp

@@ -74,7 +74,7 @@ const AP_Param::GroupInfo AP_Camera::var_info[] = {
  
     // @Param: FEEDBACK_PIN
     // @DisplayName: Camera feedback pin
-    // @Description: pin number to use for save accurate camera feedback messages. If set to -1 then don't use a pin flag for this, otherwise this is a pin number which if held high after a picture trigger order, will save camera messages when camera really takes a picture. An universal camera hot shoe is needed
+    // @Description: pin number to use for save accurate camera feedback messages. If set to -1 then don't use a pin flag for this, otherwise this is a pin number which if held high after a picture trigger order, will save camera messages when camera really takes a picture. A universal camera hot shoe is needed. The pin should be held high for at least 2 milliseconds for reliable trigger detection.
     // @Values: -1:Disabled, 0-8:APM FeedbackPin, 50-55:PixHawk FeedbackPin
     // @User: Standard
     AP_GROUPINFO("FEEDBACK_PIN",  8, AP_Camera, _feedback_pin, AP_CAMERA_FEEDBACK_DEFAULT_FEEDBACK_PIN),