ModelChoiceField.queryset documentation isn't clear that this is lazily evaluated

[Nick Smith]

The ​comments in ModelChoiceField._get_choices suggest that the queryset attribute is evaluated afresh every time the form is initialised, and testing confirms this:

1. load form
2. edit eligible related objects in the db
3. refresh form (without restarting the server)
4. observe field choices have changed

22841 also supports this theory, (although the rather old 18272 suggests otherwise).

The syntax for a field of this type is:

    foo = forms.ModelChoiceField(
        queryset=Bar.objects.filter(baz=True)
    )

Given that this looks very like a runtime-evaluated queryset, I suggest a docs amendment that clarifies the behaviour. (I am not confident in making that docs amendment myself, at least until I have properly understood the intended behaviour.)