Improve comments

Author: odersky

src/dotty/tools/dotc/transform/Mixin.scala

@@ -47,10 +47,23 @@ import collection.mutable
  *        reverse linearization order, add the following definitions to C:
  *
  *          3.1 (done in `traitInits`) For every concrete trait getter `<mods> def x(): T` in M,
- *              in order of textual occurrence:
+ *              in order of textual occurrence, produce the following:
+ *
+ *              3.1.1 If `x` is also a member of `C`, and M is a Dotty trait:
  *
  *                <mods> def x(): T = super[M].initial$x()
  *
+ *              3.1.2 If `x` is also a member of `C`, and M is a Scala 2.x trait:
+ *
+ *                <mods> def x(): T = _
+ *
+ *              3.1.3 If `x` is not a member of `C`, and M is a Dotty trait:
+ *
+ *                super[M].initial$x()
+ *
+ *              3.1.4 If `x` is not a member of `C`, and M is a Scala2.x trait, nothing gets added.
+ *
+ *
  *          3.2 (done in `superCallOpt`) The call:
  *
  *                super[M].<init>

src/dotty/tools/dotc/transform/TraitConstructors.scala

@@ -23,7 +23,6 @@ class TraitConstructors extends MiniPhaseTransform with SymTransformer {
 
   def transformSym(sym: SymDenotation)(implicit ctx: Context): SymDenotation = {
     if (sym.isPrimaryConstructor && (sym.owner is Flags.Trait))
-      // TODO: Someone needs to carefully check if name clashes are possible with this mangling scheme
       sym.copySymDenotation(name = nme.IMPLCLASS_CONSTRUCTOR)
     else sym
   }