Document vars behavior when __dict__ is missing (GH-21466) (GH-21939)

(cherry picked from commit 802726acf6048338394a6a4750835c2cdd6a947b)

Co-authored-by: Andre Delfino <adelfino@gmail.com>
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index d0f6072..b88c56e 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -1720,6 +1720,9 @@
    locals dictionary is only useful for reads since updates to the locals
    dictionary are ignored.
 
+   A :exc:`TypeError` exception is raised if an object is specified but
+   it doesn't have a :attr:`~object.__dict__` attribute (for example, if
+   its class defines the :attr:`~object.__slots__` attribute).
 
 .. function:: zip(*iterables)