Fixed #21614 -- improved development section of email docs

This patch improves the section of the email docs about [configuring email for development](https://docs.djangoproject.com/en/dev/topics/email/#testing-email-sending). It clarifies that the section is not so much about testing email as it is about configuring email for development. It also makes 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).