SI-5730 hide constructors of sealed abstract classes in scaladoc

Sealed abstract classes (like `List`) have a primary constructor, public
by default. It can never be called by external code but it shows up in
the scaladoc as a nice `new List()` construtor...

If a class is only abstract, the constructor is still useful because
people can subclass and call it. If it is only sealed (i.e. final), then
it is the normal constructor of a final class. But sealed *and* abstract
makes documenting the constructor useless.

This should remove the misleading constructor of `List`, `Double`,
'Option` and others from the scaladoc.