Merge pull request #9947 from sovrasov:floodfill_docs

modules/imgproc/include/opencv2/imgproc.hpp

@@ -3571,7 +3571,8 @@ taller than image. Since this is both an input and output parameter, you must ta
 of initializing it. Flood-filling cannot go across non-zero pixels in the input mask. For example,
 an edge detector output can be used as a mask to stop filling at edges. On output, pixels in the
 mask corresponding to filled pixels in the image are set to 1 or to the a value specified in flags
-as described below. It is therefore possible to use the same mask in multiple calls to the function
+as described below. Additionally, the function fills the border of the mask with ones to simplify
+internal processing. It is therefore possible to use the same mask in multiple calls to the function
 to make sure the filled areas do not overlap.
 @param seedPoint Starting point.
 @param newVal New value of the repainted domain pixels.