doc improvement.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@3378 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/model/User.java

@@ -255,7 +255,8 @@ public class User extends AbstractModelObject {
 
 
     /**
-     * Keyed by {@link User#id}.
+     * Keyed by {@link User#id}. This map is used to ensure
+     * singleton-per-id semantics of {@link User} objects.
      */
     private static final Map<String,User> byName = new HashMap<String,User>();