Document `ClassicalForms_InvariantFormFrobenius` and `ClassicalForms_GeneratorsWithoutScalarsDual`

[SoongNoonien]

As noted in gap-packages/recog#7 by @fingolfin the recog package uses these methods while they aren't part of the documentation so it is not guaranteed that they will remain functioning as they are right now.

@jdebeule Wanted to have an issue here to be remained of this situation. But I'm not sure if this is still the case :-)

[fingolfin]

Thank you, that's a helpful to have here as a reminder.

Whether we document it or not, we definitely need to make sure that we don't remove or rename these functions, nor break their APIs.