Document format limitations of ImageStat.Stat.extrema (#3661)

Document format limitations of ImageStat.Stat.extrema

docs/reference/ImageStat.rst

@@ -20,6 +20,13 @@ for a region of an image.
 
         Min/max values for each band in the image.
 
+        .. Note:: This relies on the :py:meth:`~PIL.Image.histogram` method, and simply
+        returns the low and high bins used. This is correct for images with 8 bits per
+        channel, but fails for other modes such as ``I`` or ``F``. Instead, use
+        :py:meth:`~PIL.Image.getextrema` to return per-band extrema for the image.
+        This is more correct and efficient because, for non-8-bit modes, the histogram
+        method uses :py:meth:`~PIL.Image.getextrema` to determine the bins used.
+
     .. py:attribute:: count
 
         Total number of pixels for each band in the image.