TrainData doc update

Update docs according to PR #5127

modules/ml/include/opencv2/ml.hpp

@@ -252,6 +252,8 @@ public:
     @param missch The character used to specify missing measurements. It should not be a digit.
         Although it's a non-numerical value, it surely does not affect the decision of whether the
         variable ordered or categorical.
+    @note If the dataset only contains input variables and no responses, use responseStartIdx = -2
+        and responseEndIdx = 0. The output variables vector will just contain zeros.
      */
     static Ptr<TrainData> loadFromCSV(const String& filename,
                                       int headerLineCount,