message: Document imwrite depth fallback
diff: @@ -212,6 +212,8 @@ can be saved using this function, with these exceptions:
  should have alpha set to 0, fully opaque pixels should have alpha set to 255/65535 (see the code sample below).
  - Multiple images (vector of Mat) can be saved in TIFF format (see the code sample below).
  +If the image format is not supported, the image will be converted to 8-bit unsigned (CV_8U) and saved that way.
  +
  If the format, depth or channel order is different, use
  Mat::convertTo and cv::cvtColor to convert it before saving. Or, use the universal FileStorage I/O
  functions to save the image to XML or YAML format.
  