diff --git a/docs/internals/deprecation.txt b/docs/internals/deprecation.txt
index 603f20dbab7..91fde966fca 100644
--- a/docs/internals/deprecation.txt
+++ b/docs/internals/deprecation.txt
@@ -114,6 +114,9 @@ these changes.
   beyond that of a simple ``TextField`` since the removal of oldforms.
   All uses of ``XMLField`` can be replaced with ``TextField``.
 
+* The undocumented ``mixin`` parameter to the ``open()`` method of
+  ``django.core.files.storage.Storage`` (and subclasses) will be removed.
+
 
 1.5
 ---
diff --git a/docs/releases/1.4.txt b/docs/releases/1.4.txt
index ea7038430ce..0c0c726d450 100644
--- a/docs/releases/1.4.txt
+++ b/docs/releases/1.4.txt
@@ -780,9 +780,10 @@ removed.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The ``open`` method of the base Storage class took an obscure parameter
-``mixin`` which allowed to dynamically change the base classes of the
-returned file object. In the rare case you relied on the `mixin` parameter,
-you can easily achieve the same by overriding the `open` method, e.g.::
+``mixin`` which allowed you to dynamically change the base classes of the
+returned file object. This has been removed. In the rare case you relied on the
+`mixin` parameter, you can easily achieve the same by overriding the `open`
+method, e.g.::
 
     from django.core.files import File
     from django.core.files.storage import FileSystemStorage