Improve docstring on order of transformations (#7248)

* Improve docstring on order of transformations

* Add code markers and be more precise

keras/preprocessing/image.py

@@ -365,8 +365,9 @@ class ImageDataGenerator(object):
         horizontal_flip: whether to randomly flip images horizontally.
         vertical_flip: whether to randomly flip images vertically.
         rescale: rescaling factor. If None or 0, no rescaling is applied,
-            otherwise we multiply the data by the value provided
-            (before applying any other transformation).
+            otherwise we multiply the data by the value provided. This is
+            applied after the `preprocessing_function` (if any provided)
+            but before any other transformation.
         preprocessing_function: function that will be implied on each input.
             The function will run before any other modification on it.
             The function should take one argument: