Update docs for img_as_ubyte

The line in `img_as_ubyte` needs correction; ubyte cannot hold negative data. From a quick experiment it seems like both `img_as_uint` and `img_as_ubyte` clip negative values (and not shift them to the positive domain).
I also find this confusing, because the output image will have only positive values *also* if the input data is negative. Why mention this?