Reword reference article about type projection so that it highlights what is allowed (#22793)

Was previously focused on what was forbidden/dangerous which made it
feel like an unsafe feature

See for example:
https://users.scala-lang.org/t/path-dependent-types-accessing-outer-class/10620/8

I left the paragraph about "type-level encoding of a combinator
calculus" but it feels more like trivia than useful information ?

Author: odersky

docs/_docs/reference/dropped-features/type-projection.md

@@ -4,15 +4,18 @@ title: "Dropped: General Type Projection"
 nightlyOf: https://docs.scala-lang.org/scala3/reference/dropped-features/type-projection.html
 ---
 
-Scala so far allowed general type projection `T#A` where `T` is an arbitrary type
-and `A` names a type member of `T`.
+Scala 2 allowed general type projection `T#A` where `T` is an arbitrary type and `A` names a type member of `T`.
+This turns out to be [unsound](https://github.com/scala/scala3/issues/1050) (at least when combined with other Scala 3 features).
 
-Scala 3 disallows this if `T` is an abstract type (class types and type aliases
-are fine). This change was made because unrestricted type projection
-is [unsound](https://github.com/scala/scala3/issues/1050).
-
-This restriction rules out the [type-level encoding of a combinator
-calculus](https://michid.wordpress.com/2010/01/29/scala-type-level-encoding-of-the-ski-calculus/).
+To remedy this, Scala 3 only allows type projection if `T` is a concrete type (any type which is not abstract), an example for such a type would be a class type (`class T`).
+A type is abstract if it is:
+* An abstract type member (`type T` without `= SomeType`)
+* A type parameter (`[T]`)
+* An alias to an abstract type (`type T = SomeAbstractType`).
+There are no restriction on `A` apart from the fact it has to be a member type of `T`, for example a subclass (`class T { class A }`).
 
 To rewrite code using type projections on abstract types, consider using
 path-dependent types or implicit parameters.
+
+This restriction rules out the [type-level encoding of a combinator
+calculus](https://michid.wordpress.com/2010/01/29/scala-type-level-encoding-of-the-ski-calculus/).