Doc fix: duplicate object description of email.message (GH-13742)

diff --git a/Doc/library/email.compat32-message.rst b/Doc/library/email.compat32-message.rst
index f02b35b..09ea64a 100644
--- a/Doc/library/email.compat32-message.rst
+++ b/Doc/library/email.compat32-message.rst
@@ -6,6 +6,7 @@
 .. module:: email.message
    :synopsis: The base class representing email messages in a fashion
               backward compatible with Python 3.2
+   :noindex:
 
 
 The :class:`Message` class is very similar to the