Add a comment for the default value of the rename function.

[Cherry-picked b23da16]

Author: noti0na1

compiler/src/dotty/tools/dotc/core/Scopes.scala

@@ -159,6 +159,7 @@ object Scopes {
 
     /** The scope that keeps only those symbols from this scope that match the
      *  given predicates, renamed with the given rename function.
+     *  If renaming is not needed for a symbol, the rename function should return `null`.
      *  If all symbols match and none are renamed, returns the scope itself, otherwise
      *  a copy with the matching and renamed symbols.
      */