Fixed #23305 -- Added documentation item

Warned against importing models in __init__.py

This PR addresses this Trac ticket: https://code.djangoproject.com/ticket/23305

Although this doc modification might not be a perfect fit in applications.txt, it seemed better than other alternatives.

I tested this locally, which generated this html when doing a `make html`:
```
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Additionally, avoid importing models in __init__.py; doing so leads to
inconsistent behavior.</p>
</div>
```