Improve the doc for cv::Mat::checkVector.

modules/core/include/opencv2/core/mat.hpp

@@ -1786,7 +1786,27 @@ public:
      */
     size_t total(int startDim, int endDim=INT_MAX) const;
 
-    //! returns N if the matrix is 1-channel (N x ptdim) or ptdim-channel (1 x N) or (N x 1); negative number otherwise
+    /**
+     * @param elemChannels Number of channels or number of columns the matrix should have.
+     *                     For a 2-D matrix, when the matrix has only 1 column, then it should have
+     *                     elemChannels channels; When the matrix has only 1 channel,
+     *                     then it should have elemChannels columns.
+     *                     For a 3-D matrix, it should have only one channel. Furthermore,
+     *                     if the number of planes is not one, then the number of rows
+     *                     within every plane has to be 1; if the number of rows within
+     *                     every plane is not 1, then the number of planes has to be 1.
+     * @param depth The depth the matrix should have. Set it to -1 when any depth is fine.
+     * @param requireContinuous Set it to true to require the matrix to be continuous
+     * @return -1 if the requirement is not satisfied.
+     *         Otherwise, it returns the number of elements in the matrix. Note
+     *         that an element may have multiple channels.
+     *
+     * The following code demonstrates its usage for a 2-d matrix:
+     * @snippet snippets/core_mat_checkVector.cpp example-2d
+     *
+     * The following code demonstrates its usage for a 3-d matrix:
+     * @snippet snippets/core_mat_checkVector.cpp example-3d
+     */
     int checkVector(int elemChannels, int depth=-1, bool requireContinuous=true) const;
 
     /** @brief Returns a pointer to the specified matrix row.