Documentation - Add example for reverse filtering

[chgad]

I came across the case where I wanted to query all Tags which have a certain property set on the related model like:

>>>Tag.objects.filter(food__is_vegetarian=True)
[<Tag: salad>, <Tag: salad>, <Tag: fruit>, <Tag: fruit>]

which can be easily solved by the given example for the Food query here by adding the .distinct() call.

Once you think about it, it is obvious but maybe we can just add it to be more explicit on this regard.