[JENKINS-755] Describe the built-in JDK as '(System)'.

As suggested in
https://issues.jenkins-ci.org/browse/JENKINS-755?focusedCommentId=206175&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-206175
this avoids confusion where users incorrectly think they're getting a
JDK they've listed in the configuration page.

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

@@ -54,10 +54,10 @@ import org.kohsuke.stapler.DataBoundConstructor;
 public final class JDK extends ToolInstallation implements NodeSpecific<JDK>, EnvironmentSpecific<JDK> {
 
     /**
-     * Name of the “default JDK”, meaning no specific JDK selected.
+     * Name of the “System JDK”, which is just the JDK on Jenkins' $PATH.
      * @since 1.577
      */
-    public static final String DEFAULT_NAME = "(Default)";
+    public static final String DEFAULT_NAME = "(System)";
 
     /**
      * @deprecated since 2009-02-25