Improve boundary doc comment

Author: odersky

library/src/scala/util/boundary.scala

@@ -33,6 +33,11 @@ object boundary:
     /*message*/ null, /*cause*/ null, /*enableSuppression=*/ false, /*writableStackTrace*/ false)
 
   /** Labels are targets indicating which boundary will be exited by a `break`.
+   *  A Label is generated and passed to code wrapped in a `boundary` call. Example
+   *
+   *    boundary[Unit]:
+   *      ...
+   *      summon[Label[Unit]] // will link to the label generated by `boundary`
    */
   final class Label[-T]