Add comment explaining why ErrorKind cannot FTTB be an enum

Author: anatoliykmetyuk

library/src/scala/compiletime/testing/ErrorKind.scala

@@ -1,7 +1,8 @@
 package scala.compiletime.testing
 
 /** An error can be either a parse-time or a typecheck-time */
-sealed trait ErrorKind  // TODO make this enum, so far not doable because ScalaJS compilation fails on it
+sealed trait ErrorKind // This should be an enum but currently, Dotty lib fails to
+                       // compile with an obscure error.
 object ErrorKind:
   case object Parser extends ErrorKind
   case object Typer extends ErrorKind