fixup! changes: Add draft entry for loadAt/loadBeforeEx/DemoStorage fix

@d-maurer says (https://github.com/zopefoundation/ZODB/pull/323#discussion_r627929959):

The changes around `loadBeforeEx` have much more impact than the `DemoStorage` fix.

--------

kirr: adjusted the text a bit: "Introduces" -> "Introduce"; add
"interface". @d-maurer, I hope it is ok.

CHANGES.rst

@@ -5,9 +5,11 @@
 5.6.1 (unreleased)
 ==================
 
-- Fix DemoStorage data corruption in the presence of whiteouts.
-  This change is implemented via introducing new ``loadBeforeEx`` interface
-  that extends and corrects ``loadBefore`` semantic.
+- Introduce a new ``loadBeforeEx`` interface and deprecate ``loadBefore``:
+  ``loadBeforeEx`` is simpler, provides better information for object delete
+  records and can be more efficiently implemented by many storages.
+  ``loadBeforeEx`` is used (and required) to fix a ``DemoStorage`` data corruption
+  in the presence of object delete records.
   See `issue 318 <https://github.com/zopefoundation/ZODB/issues/318>`_
   and `PR 323 <https://github.com/zopefoundation/ZODB/pull/323>`_
   for details.