Issue #7119: document that a program-generated Message object
tree may be mutated when serialized.
diff --git a/Doc/library/email.mime.rst b/Doc/library/email.mime.rst
index 94df8f0..10f3e37 100644
--- a/Doc/library/email.mime.rst
+++ b/Doc/library/email.mime.rst
@@ -71,7 +71,8 @@
also be added.
Optional *boundary* is the multipart boundary string. When ``None`` (the
- default), the boundary is calculated when needed.
+ default), the boundary is calculated when needed (for example, when the
+ message is serialized).
*_subparts* is a sequence of initial subparts for the payload. It must be
possible to convert this sequence to a list. You can always attach new subparts