Document a few details about class

[Morriar]

Following the discussion in #2139 (comment).

[ParadoxV5]

class refers to the class itself. From my understanding,

class A
  X: class # A
end
class B < A
  # B::X is of type B
end

---

I’ve always considered instance and self to be different:

SUBJECT = Object.new
SUBJECT.extend Comparable # i.e., SUBJECT.singleton_class.include Comparable

class Object
  def dup: () -> instance # Object
  def itself: () -> self # Object
end
SUBJECT: Object & Comparable
# SUBJECT.dup    -> Object
# SUBJECT.itself -> Object & Comparable