update documentation of recheck about top-level boxes

Author: Linyxus

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -520,7 +520,8 @@ class CheckCaptures extends Recheck, SymTransformer:
         case _: RefTree => true
         case _          => false
 
-    /** If expected type `pt` is boxed, don't propagate free variables.
+    /** If expected type `pt` is boxed and the tree is a function or a reference,
+     *  don't propagate free variables.
      *  Otherwise, if the result type is boxed, simulate an unboxing by
      *  adding all references in the boxed capture set to the current environment.
      */