message: clarify opencl execution context doc
  fix opencv/opencv#18888
diff: @@ -743,9 +743,11 @@ public:
  /** Get associated ocl::Context */
  Context& getContext() const;
  - /** Get associated ocl::Device */
  + /** Get the single default associated ocl::Device */
  Device& getDevice() const;
  - /** Get associated ocl::Queue */
  + /** Get the single ocl::Queue that is associated with the ocl::Context and
  + * the single default ocl::Device
  + */
  Queue& getQueue() const;
  bool useOpenCL() const;
  