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.)