bpo-15115: Document deprecation of email.encoders in Python 3 (GH-5354)

Doc/library/email.encoders.rst

@@ -12,6 +12,11 @@ This module is part of the legacy (``Compat32``) email API.  In the
 new API the functionality is provided by the *cte* parameter of
 the :meth:`~email.message.EmailMessage.set_content` method.
 
+This module is deprecated in Python 3.  The functions provided here
+should not be called explicitly since the :class:`~email.mime.text.MIMEText`
+class sets the content type and CTE header using the *_subtype* and *_charset*
+values passed during the instaniation of that class.
+
 The remaining text in this section is the original documentation of the module.
 
 When creating :class:`~email.message.Message` objects from scratch, you often