specified KeyPoint::angle in rst doc and code comment

modules/features2d/doc/common_interfaces_of_feature_detectors.rst

@@ -24,7 +24,7 @@ KeyPoint
 
   .. ocv:member:: float angle
 
-     computed orientation of the keypoint (-1 if not applicable)
+     computed orientation of the keypoint (-1 if not applicable). Its possible values are in a range [0,360) degrees. It is measured relative to image coordinate system (y-axis is directed downward), ie in clockwise.
 
   .. ocv:member:: float response
 

modules/features2d/include/opencv2/features2d/features2d.hpp

@@ -100,7 +100,9 @@ public:
 
     CV_PROP_RW Point2f pt; //!< coordinates of the keypoints
     CV_PROP_RW float size; //!< diameter of the meaningful keypoint neighborhood
-    CV_PROP_RW float angle; //!< computed orientation of the keypoint (-1 if not applicable)
+    CV_PROP_RW float angle; //!< computed orientation of the keypoint (-1 if not applicable);
+							//!< it's in [0,360) degrees and measured relative to 
+							//!< image coordinate system, ie in clockwise. 
     CV_PROP_RW float response; //!< the response by which the most strong keypoints have been selected. Can be used for the further sorting or subsampling
     CV_PROP_RW int octave; //!< octave (pyramid layer) from which the keypoint has been extracted
     CV_PROP_RW int class_id; //!< object class (if the keypoints need to be clustered by an object they belong to)