Ticket no. 20183 - documentation note about existing data

This is fix for ticket no. 20183 (https://code.djangoproject.com/ticket/20183).

Generally documentation pages suggest the code should work as-is, but this breaks on one of the documentation pages. Some earlier example mentions that specific instance has already been created, but in this case there is no note, and adding full creation code would blur the examples (according to [this comment](https://code.djangoproject.com/ticket/20183#comment:6)).

The fix is simply to notify the user that the instances in question have already been saved in the database. If the fix is not enough, it is possible to rewrite the example to use models with less required fields.