Document which constraints model formsets validate across forms

[Clifford Gama]

Model formsets validate unique, unique_together, unique_for_date|month|year, and UniqueConstraints with fields and no condition across forms in the formset (#9493, #30581). This is only mentioned in passing, in ​Overriding clean() on a ModelFormSet, as the reason to call super().clean(). That passage lists only the first three. The UniqueConstraint support added in #30581 was never documented for formsets.

The docs also don't say:

• that violation_error_message isn't used for cross-form errors; a generic "Please correct the duplicate data for …" message is shown instead;
• that other constraints (conditional or expression-based UniqueConstraints, ExclusionConstraint) are checked only against existing rows, not across forms, so saving can raise IntegrityError.

​PR

Thanks Tim Schilling for the nudge to revive this work.