Merged revisions 77417 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/branches/py3k

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  r77417 | r.david.murray | 2010-01-10 14:18:27 -0500 (Sun, 10 Jan 2010) | 10 lines

  Merged revisions 77412 via svnmerge from
  svn+ssh://pythondev@svn.python.org/python/trunk

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    r77412 | r.david.murray | 2010-01-10 12:41:28 -0500 (Sun, 10 Jan 2010) | 3 lines

    Issue #7119: document that a program-generated Message object
    tree may be mutated when serialized.
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diff --git a/Doc/library/email.message.rst b/Doc/library/email.message.rst
index 93dda69..7e939c9 100644
--- a/Doc/library/email.message.rst
+++ b/Doc/library/email.message.rst
@@ -40,7 +40,10 @@
 
       Return the entire message flattened as a string.  When optional *unixfrom*
       is ``True``, the envelope header is included in the returned string.
-      *unixfrom* defaults to ``False``.
+      *unixfrom* defaults to ``False``.  Flattening the message may trigger
+      changes to the :class:`Message` if defaults need to be filled in to
+      complete the transformation to a string (for example, MIME boundaries may
+      be generated or modified).
 
       Note that this method is provided as a convenience and may not always
       format the message the way you want.  For example, by default it mangles