Merge #11216: document all possible set_charset execution paths.
diff --git a/Doc/library/email.message.rst b/Doc/library/email.message.rst
index 6f40c05..a570d81 100644
--- a/Doc/library/email.message.rst
+++ b/Doc/library/email.message.rst
@@ -133,8 +133,22 @@
string naming a character set, or ``None``. If it is a string, it will
be converted to a :class:`~email.charset.Charset` instance. If *charset*
is ``None``, the ``charset`` parameter will be removed from the
- :mailheader:`Content-Type` header. Anything else will generate a
- :exc:`TypeError`.
+ :mailheader:`Content-Type` header (the message will not be otherwise
+ modified). Anything else will generate a :exc:`TypeError`.
+
+ If there is no existing :mailheader:`MIME-Version` header one will be
+ added. If there is no existing :mailheader:`Content-Type` header, one
+ will be added with a value of :mimetype:`text/plain`. Whether the
+ :mailheader:`Content-Type` header already exists or not, its ``charset``
+ parameter will be set to *charset.output_charset*. If
+ *charset.input_charset* and *charset.output_charset* differ, the payload
+ will be re-encoded to the *output_charset*. If there is no existing
+ :mailheader:`Content-Transfer-Encoding` header, then the payload will be
+ transfer-encoded, if needed, using the specified
+ :class:`~email.charset.Charset`, and a header with the appropriate value
+ will be added. If a :mailheader:`Content-Transfer-Encoding` header
+ already exists, the payload is assumed to already be correctly encoded
+ using that :mailheader:`Content-Transfer-Encoding` and is not modified.
The message will be assumed to be of type :mimetype:`text/\*`, with the
payload either in unicode or encoded with *charset.input_charset*.