bpo-28974: `object.__format__(x, '')` is now equivalent to `str(x)` (#506)

rather than `format(str(self), '')`.
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 25afc35..36bd71a 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1282,6 +1282,10 @@
       The __format__ method of ``object`` itself raises a :exc:`TypeError`
       if passed any non-empty string.
 
+   .. versionchanged:: 3.7
+      ``object.__format__(x, '')`` is now equivalent to ``str(x)`` rather
+      than ``format(str(self), '')``.
+
 
 .. _richcmpfuncs:
 .. method:: object.__lt__(self, other)