Fixed #18731 -- Added an example about customizing "makemessages" comman...

...d.
If I remember correctly, we don't have a formal rule for this in the [devguide](https://docs.python.org/devguide/documenting.html). I usually use section headings in the release notes.

Perhaps, we can use this in https://docs.djangoproject.com/en/dev/ref/django-admin/#makemessages?

Something like:

```rst
.. seealso:: :ref:`customizing-makemessages`
```