fix three warning about mog, mog2 and KNearestNeighbour.

modules/ocl/doc/ml_machine_learning.rst

@@ -5,7 +5,7 @@ ml.Machine Learning
 
 ocl::KNearestNeighbour
 --------------------------
-.. ocv:class:: ocl::KNearestNeighbour
+.. ocv:class:: ocl::KNearestNeighbour : public ocl::CvKNearest
 
 The class implements K-Nearest Neighbors model as described in the beginning of this section.
 

modules/ocl/doc/video_analysis.rst

@@ -361,7 +361,7 @@ Computes a background image.
 
 ocl::MOG
 ------------
-.. ocv:class:: ocl::MOG
+.. ocv:class:: ocl::MOG : public ocl::BackgroundSubtractor
 
 Gaussian Mixture-based Backbround/Foreground Segmentation Algorithm. ::
 
@@ -441,7 +441,7 @@ Releases all inner buffer's memory.
 
 ocl::MOG2
 -------------
-.. ocv:class:: ocl::MOG2
+.. ocv:class:: ocl::MOG2 : public ocl::BackgroundSubtractor
 
 Gaussian Mixture-based Background/Foreground Segmentation Algorithm. ::