Update GpuMat, GpuMat::download, GpuMat::upload documentation

modules/core/include/opencv2/core/cuda.hpp

@@ -91,6 +91,15 @@ aligned to a size depending on the hardware. Single-row GpuMat is always a conti
 on its destructor. The destruction order of such variables and CUDA context is undefined. GPU memory
 release function returns error if the CUDA context has been destroyed before.
 
+Some member functions are described as a "Blocking Call" while some are described as a
+"Non-Blocking Call". Blocking functions are synchronous to host. It is guaranteed that the GPU
+operation is finished when the function returns. However, non-blocking functions are asynchronous to
+host. Those functions may return even if the GPU operation is not finished.
+
+Compared to their blocking counterpart, non-blocking functions accept Stream as an additional
+argument. If a non-default stream is passed, the GPU operation may overlap with operations in other
+streams.
+
 @sa Mat
  */
 class CV_EXPORTS GpuMat
@@ -151,16 +160,38 @@ public:
     //! swaps with other smart pointer
     void swap(GpuMat& mat);
 
-    //! pefroms upload data to GpuMat (Blocking call)
+    /** @brief Performs data upload to GpuMat (Blocking call)
+
+    This function copies data from host memory to device memory. As being a blocking call, it is
+    guaranteed that the copy operation is finished when this function returns.
+    */
     void upload(InputArray arr);
 
-    //! pefroms upload data to GpuMat (Non-Blocking call)
+    /** @brief Performs data upload to GpuMat (Non-Blocking call)
+
+    This function copies data from host memory to device memory. As being a non-blocking call, this
+    function may return even if the copy operation is not finished.
+
+    The copy operation may be overlapped with operations in other non-default streams if \p stream is
+    not the default stream and \p dst is HostMem allocated with HostMem::PAGE_LOCKED option.
+    */
     void upload(InputArray arr, Stream& stream);
 
-    //! pefroms download data from device to host memory (Blocking call)
+    /** @brief Performs data download from GpuMat (Blocking call)
+
+    This function copies data from device memory to host memory. As being a blocking call, it is
+    guaranteed that the copy operation is finished when this function returns.
+    */
     void download(OutputArray dst) const;
 
-    //! pefroms download data from device to host memory (Non-Blocking call)
+    /** @brief Performs data download from GpuMat (Non-Blocking call)
+
+    This function copies data from device memory to host memory. As being a non-blocking call, this
+    function may return even if the copy operation is not finished.
+
+    The copy operation may be overlapped with operations in other non-default streams if \p stream is
+    not the default stream and \p dst is HostMem allocated with HostMem::PAGE_LOCKED option.
+    */
     void download(OutputArray dst, Stream& stream) const;
 
     //! returns deep copy of the GpuMat, i.e. the data is copied