Issue #7119: document that a program-generated Message object
tree may be mutated when serialized.
diff --git a/Doc/library/email.generator.rst b/Doc/library/email.generator.rst
index 8ba426a..f02e7d8 100644
--- a/Doc/library/email.generator.rst
+++ b/Doc/library/email.generator.rst
@@ -17,7 +17,10 @@
standards-compliant way, should handle MIME and non-MIME email messages just
fine, and is designed so that the transformation from flat text, to a message
structure via the :class:`~email.parser.Parser` class, and back to flat text,
-is idempotent (the input is identical to the output).
+is idempotent (the input is identical to the output). On the other hand, using
+the Generator on a :class:`~email.message.Message` constructed by program may
+result in changes to the :class:`~email.message.Message` object as defaults are
+filled in.
Here are the public methods of the :class:`Generator` class, imported from the
:mod:`email.generator` module: