message: imgproc: clarify usage of the mask in floodfill
diff: @@ -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.
  