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

[ezio.melotti 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`.
Is there any way to either set a shortcut somewhere for
email.message.Message or set is as current class/module?

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