SI-6566, type alias bodies are invariant: update spec

Reflect the fix to SI-6566 inside the spec. In fact, even the more relaxed behavior before SI-6566 was not documented.

Moreover, mention the exception for object-private/protected types —
the following code compiles:

trait Fun0[-S, +T] {
  protected[this] type Eval = S => T
}

Review by @adriaanm.