Fixed #8848: clarified custom file storage backend docs in light of the more clear code comments.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8942 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/howto/custom-file-storage.txt

@@ -3,6 +3,8 @@
 Writing a custom storage system
 ===============================
 
+.. currentmodule:: django.core.files.storage
+
 If you need to provide custom file storage -- a common example is storing files
 on some remote system -- you can do so by defining a custom storage class.
 You'll need to follow these steps:
@@ -35,8 +37,16 @@ You'll need to follow these steps:
    the ``path()`` method.
 
 Your custom storage system may override any of the storage methods explained in
-:ref:`ref-files-storage`. However, it's usually better to use the hooks
-specifically designed for custom storage objects. These are:
+:ref:`ref-files-storage`, but you **must** implement the following methods:
+
+    * :meth:`Storage.delete`
+    * :meth:`Storage.exists`
+    * :meth:`Storage.listdir`
+    * :meth:`Storage.size`
+    * :meth:`Storage.url`
+
+You'all also usually want to use hooks specifically designed for custom storage
+objects. These are:
 
 ``_open(name, mode='rb')``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~