When I wrote those docs I intended to add a policy argument to as_string, but did not do it by the time 3.3 was released. The docs for 3.3 need to be corrected, and I need to decide if I do want to add that keyword to as_string in 3.4 (the answer is probably yes, but I may check with the email-sig first).
Turns out even with as_string accepting a policy keyword, the example still failed. It needs to be generating a bytes object, not a string. Before I even realized that, though, I decided I wanted to add as_bytes (and __bytes__). So the attached patch does that: adds policy to as_string, and adds new methods as_bytes and __bytes__ to Message. This patch is only for 3.4. The 3.3 patch will just delete that part of the policy example, so I'm not going to bother to upload it.
