Add more details page on Class Shadowing

Author: OlivierBlanvillain

docs/docs/reference/dropped/class-shadowing-spec.md

@@ -0,0 +1,25 @@
+---
+layout: doc-page
+title: "Dropped: Class Shadowing - More Details"
+---
+
+Spec diff: in section [5.1.4 Overriding](https://www.scala-lang.org/files/archive/spec/2.12/05-classes-and-objects.html), add *M' must not be a class*.
+
+> Why do we want to make this change to the language?
+
+tbd
+
+
+> How much existing code is going to be affected?
+
+From all the code compiled so far with Dotty the only instance of this I could find is in the stdlib. Looking at [this commit](https://github.com/lampepfl/scala/commit/68f13bf39979b631ed211ec1751934306ceb5d6c#diff-7aa508b70e055b47c823764e3e5646b8) it seems like the usage of class shadowing was accidental.
+
+
+> How exactly is existing code going to be affected?
+
+Code that relies on overridden inner classes will stop compiling.
+
+
+> Is this change going to be migratable automatically?
+
+No.

docs/docs/reference/dropped/class-shadowing.md

@@ -24,6 +24,4 @@ other, but classes in Scala cannot be overridden. To keep things clean
 (and its internal operations consistent) the Dotty compiler forces you
 to rename the inner classes so that their names are different.
 
-
-
-
+[More details](./class-shadowing-spec.html)