[docs] Fix internal doc references for the email package (issue 18761)

[rdmurray at bitdance.com]

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.errors.rst
File Doc/library/email.errors.rst (left):

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.errors.rst#oldcode50
Doc/library/email.errors.rst:50: :meth:`add_payload`, but the payload is
already a scalar and the message's
add_payload no longer exists.

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.headerregistry.rst
File Doc/library/email.headerregistry.rst (left):

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.headerregistry.rst#oldcode59
Doc/library/email.headerregistry.rst:59: is exactly the value passed in
the :attr:`~EmailPolicy.header_factory`
Hmm.  I would have though that this would work, being in the same rst
file.  Oh well.

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.message.rst
File Doc/library/email.message.rst (right):

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.message.rst#newcode316
Doc/library/email.message.rst:316: :meth:`~object.__getitem__` except
that optional *failobj* is returned if the
This change is incorrect.  The original construct correctly links to
Message's __getitem__.

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.message.rst#newcode331
Doc/library/email.message.rst:331: Extended header setting.  This method
is similar to :meth:`~object.__setitem__`
This change is also incorrect.

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.policy.rst
File Doc/library/email.policy.rst (right):

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.policy.rst#newcode318
Doc/library/email.policy.rst:318: :mod:`email.policy` module also
defines an instance of this class,
missing ~.

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.rst
File Doc/library/email.rst (right):

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.rst#newcode174
Doc/library/email.rst:174: :class:`~email.mime.text.MIMEText`
constructor, the :meth:`Message.add_payload` method, the
Don't we need ~email.message here as well?

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.rst#newcode205
Doc/library/email.rst:205: false, :meth:`get_payload` was not a list
object, but a single :class:`~email.message.Message`
Don't we need ~email.message.Message here?

http://bugs.python.org/review/18761/diff/8959/Doc/library/email.rst#newcode217
Doc/library/email.rst:217: doesn't do a :meth:`set_payload` with a
:class:`~email.message.Message` instance on a container
~email.message.Message.set_payload?

http://bugs.python.org/review/18761/