Merge: #1525919: Document MIMEText+set_payload encoding behavior.

diff --cc Doc/library/email.mime.rst
index db5584c905aa88fba47bde8175ba9ff53af32244,2ce486820f0a4818dc6a2ed9d2a3b33599c3a4a0..acaba1ce400331866727d2dfbe18f7d3f9c3c40a
--- 1/Doc/library/email.mime.rst
--- 2/Doc/library/email.mime.rst
+++ b/Doc/library/email.mime.rst
@@@ -185,5 -185,15 +185,15 @@@ Here are the classes
     minor type and defaults to :mimetype:`plain`.  *_charset* is the character
     set of the text and is passed as a parameter to the
     :class:`~email.mime.nonmultipart.MIMENonMultipart` constructor; it defaults
 -   to ``us-ascii``.
 +   to ``us-ascii`` if the string contains only ``ascii`` codepoints, and
 +   ``utf-8`` otherwise.
+ 
+    Unless the ``_charset`` parameter is explicitly set to ``None``, the
+    MIMEText object created will have both a :mailheader:`Content-Type` header
+    with a ``charset`` parameter, and a :mailheader:`Content-Transfer-Endcoding`
+    header.  This means that a subsequent ``set_payload`` call will not result
+    in an encoded payload, even if a charset is passed in the ``set_payload``
+    command.  You can "reset" this behavior by deleting the
+    ``Content-Transfer-Encoding`` header, after which a ``set_payload`` call
+    will automatically encode the new payload (and add a new
+    :mailheader:`Content-Transfer-Encoding` header).
 -