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

[storchaka at gmail.com]

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#newcode300
Doc/library/email.rst:300: * The method :meth:`setboundary` was renamed
to :meth:`~email.message.Message.set_boundary`.
On 2013/08/16 23:38:47, ezio.melotti wrote:
> Is there any way to either set a shortcut somewhere for
email.message.Message or
> set is as current class/module?

Yes, we can change current module and class. But then we should change
them back when we want refers to entities in other modules/classes. And
we should always remember which module and class are current (it can be
declared several pages above). I think explicitness is better here.

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