DOC: Add example of bad dtype conversion.

doc/source/user_guide/data_types.txt

@@ -30,7 +30,14 @@ but, for efficiency, *may return an image of a different dtype* (see `Output
 types`_). If you need a particular dtype, ``skimage`` provides utility
 functions that convert dtypes and properly rescale image intensities (see
 `Input types`_). You should **never use** ``astype`` on an image, because it
-violates these assumptions about the dtype range.
+violates these assumptions about the dtype range::
+
+   >>> from skimage import img_as_float
+   >>> image = np.arange(0, 50, 10, dtype=np.uint8)
+   >>> print image.astype(np.float) # These float values are out of range.
+   [  0.  10.  20.  30.  40.]
+   >>> print img_as_float(image)
+   [ 0.          0.03921569  0.07843137  0.11764706  0.15686275]
 
 
 Input types