Clarified floodfill documentation

Clarified what value floodfill sets pixels in the mask to when FOODFILL_MASK_ONLY is set.

modules/imgproc/doc/miscellaneous_transformations.rst

@@ -507,7 +507,7 @@ Fills a connected component with the given color.
 
             * **FLOODFILL_FIXED_RANGE** If set, the difference between the current pixel and seed pixel is considered. Otherwise, the difference between neighbor pixels is considered (that is, the range is floating).
 
-            * **FLOODFILL_MASK_ONLY**  If set, the function does not change the image ( ``newVal``  is ignored), but fills the mask.  The flag can be used for the second variant only.
+            * **FLOODFILL_MASK_ONLY**  If set, the function does not change the image ( ``newVal``  is ignored), but fills the mask with the value in bits 8-16 of ``flags`` (that is, the fill value is set to newValue by adding (newValue << 8) to the ``flags``).  The flag can be used for the second variant only.
 
 The functions ``floodFill`` fill a connected component starting from the seed point with the specified color. The connectivity is determined by the color/brightness closeness of the neighbor pixels. The pixel at
 :math:`(x,y)` is considered to belong to the repainted domain if: