Label transpose operation for clarity

doc/source/user_guide/numpy_images.txt

@@ -174,7 +174,7 @@ in a different order, even if the number of operations is the same:
     >>> t0 = time.time(); x = in_order_multiply(im3d, 5); t1 = time.time()
     >>> print("%.2f seconds" % (t1 - t0))
     0.14 seconds
-    >>> im3d_t = np.transpose(im3d).copy()
+    >>> im3d_t = np.transpose(im3d).copy() # place "frames" dimension at end
     >>> im3d_t.shape
     (1024, 1024, 100)
     >>> s0 = time.time(); x = out_of_order_multiply(im3d, 5); s1 = time.time()