Thanks for the patch. A couple of comments:
- it would be nice to have some tests, but of course there are no tests for SMTP_SSL currently. If you are motivated, perhaps you could investigate into adding some.
- there are some tab characters in your patch. Please only use spaces for indentation (see PEP 8 if you have any doubts); make sure your editor is configured for that :)
- starttls() should probably get a context parameter too. You can take a look at imaplib for an example.
- you need to update the API docs in Doc/library/smtplib.rst

I don't think an example of using a context is necessary. By using proper ReST markup a link will be generated to the SSLContext documentation, and that's probably enough for people to understand.