message: Add docstring for UMat::handle
diff: @@ -2464,6 +2464,10 @@ public:
  UMat& operator = (UMat&& m);
  #endif
  + /*! Returns the OpenCL buffer handle on which UMat operates on.
  + The UMat instance should be kept alive during the use of the handle to prevent the buffer to be
  + returned to the OpenCV buffer pool.
  + */
  void* handle(int accessFlags) const;
  void ndoffset(size_t* ofs) const;
  