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.

New changeset 53287858e71f by R David Murray in branch 'default':
#18600: add policy to add_string, and as_bytes and __bytes__ methods.
http://hg.python.org/cpython/rev/53287858e71f
New changeset 8fbaf4e649af by R David Murray in branch '3.3':
#18600: In 3.3, as_string does not accept a policy keyword.
http://hg.python.org/cpython/rev/8fbaf4e649af
New changeset b9a5b7e3b32f by R David Murray in branch 'default':
Null merge related to #18600.
http://hg.python.org/cpython/rev/b9a5b7e3b32f

Committed with changes pointed out by Antoine (thank you Antoine).

stage: patch review -> resolved

stage: needs patch -> patch review