Fixed #21614 -- improve doc section on testing email locally.

This section improves the section on [testing email locally](https://docs.djangoproject.com/en/dev/topics/email/#testing-email-sending) by making it clearer that the section is about testing emailing locally (rather than testing the sending of email in general).  It also adds other minor improvements like adding a link to the section on unit-testing the sending of email, which is not covered in this section at all.  See [ticket #21614](https://code.djangoproject.com/ticket/21614).