Issue #7119: document that a program-generated Message object
tree may be mutated when serialized.
diff --git a/Doc/library/email.message.rst b/Doc/library/email.message.rst
index 0e79b6c..85a5d83 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