added documentation for gpu::BroxOpticalFlow, updated other docs

modules/gpu/doc/operations_on_matrices.rst

@@ -5,6 +5,42 @@ Operations on Matrices
 
 
 
+gpu::gemm
+------------------
+Performs generalized matrix multiplication.
+
+.. ocv:function:: void gemm(const GpuMat& src1, const GpuMat& src2, double alpha, const GpuMat& src3, double beta, GpuMat& dst, int flags = 0, Stream& stream = Stream::Null())
+
+    :param src1: First multiplied input matrix that should have  ``CV_32FC1`` , ``CV_64FC1`` , ``CV_32FC2`` , or  ``CV_64FC2``  type.
+
+    :param src2: Second multiplied input matrix of the same type as  ``src1`` .
+    
+    :param alpha: Weight of the matrix product.
+
+    :param src3: Third optional delta matrix added to the matrix product. It should have the same type as  ``src1``  and  ``src2`` .
+    
+    :param beta: Weight of  ``src3`` .
+    
+    :param dst: Destination matrix. It has the proper size and the same type as input matrices.
+
+    :param flags: Operation flags:
+
+            * **GEMM_1_T** transpose  ``src1``
+            * **GEMM_2_T** transpose  ``src2``
+            * **GEMM_3_T** transpose  ``src3``
+
+    :param stream: Stream for the asynchronous version.
+            
+The function performs generalized matrix multiplication similar to the ``gemm`` functions in BLAS level 3. For example, ``gemm(src1, src2, alpha, src3, beta, dst, GEMM_1_T + GEMM_3_T)`` corresponds to
+
+.. math::
+
+    \texttt{dst} =  \texttt{alpha} \cdot \texttt{src1} ^T  \cdot \texttt{src2} +  \texttt{beta} \cdot \texttt{src3} ^T
+
+.. seealso:: :ocv:func:`gemm`
+
+
+
 gpu::transpose
 ------------------
 Transposes a matrix.