This expands the deprecation message that started to pop up in v11.0.3
after the landing of commit e148382bd0,
that deprecated the `{[key: string]: any}` type for the options property of the `FormBuilder.group` method.
It turns out that having a custom validator declared as
`{ validators: (group: FormGroup) => ValidationErrors|null }` works in practice,
but is now inferred by TS as the deprecated version of `group`
(because `FormGroup` is a subclass of `AbstractControl` that `ValidatorFn` expects).
We considered the possibility of tweaking the forms API to accept such validators,
but it turns out to generate too many changes in the framework or possible breaking changes for Angular users.
We settled for a more explicit deprecation message, elaborated with the help of @petebacondarwin.
This will hopefully help developers to understand why the deprecation warning is showing up
when they think they are already using the non-deprecated overload.
PR Close#39946
67c5fe06e6