Email docs incorrectly use :meth: for functions

[Mike Edmunds]

docs/topics/email.txt incorrectly uses Sphinx's ​:meth: role for cross references to several module-level functions (that are defined with function:: directives rather than method:: directives). It should use the ​:func: role.

Sphinx is pretty flexible about the whole thing, and the invalid :meth: references still work, but it can be confusing trying to read the docs source. (What class defines the send_mail method? Oh, none, it's actually a function.)

Example incorrect usages:

As in :meth:`~django.core.mail.send_mail`, recipients in the same…
(or)
As in :meth:`send_mail`, recipients in the same…

Correct usage:

Use :func:`send_mail` for straightforward email sending.…

Because send_mail is a function, not a method:

.. function:: send_mail(subject, message, …)

Also applies to several other email functions in that same document.

[A rule of thumb is that a :meth: reference that doesn't include a dotted ClassName.method_name is probably incorrect, unless it appears within the class's own documentation and refers to a method of that class.]