update documentation

modules/imgcodecs/include/opencv2/imgcodecs.hpp

@@ -49,6 +49,7 @@
   @defgroup imgcodecs Image file reading and writing
   @{
     @defgroup imgcodecs_c C API
+    @defgroup imgcodecs_flags Flags used for image file reading and writing
     @defgroup imgcodecs_ios iOS glue
   @}
 */
@@ -60,6 +61,9 @@ namespace cv
 //! @addtogroup imgcodecs
 //! @{
 
+//! @addtogroup imgcodecs_flags
+//! @{
+
 //! Imread flags
 enum ImreadModes {
        IMREAD_UNCHANGED            = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped). Ignore EXIF orientation.
@@ -130,6 +134,8 @@ enum ImwritePAMFlags {
        IMWRITE_PAM_FORMAT_RGB_ALPHA = 5,
      };
 
+//! @} imgcodecs_flags
+
 /** @brief Loads an image from a file.
 
 @anchor imread