doc update

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@10853 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/PluginWrapper.java

@@ -464,6 +464,10 @@ public final class PluginWrapper {
     /**
      * If the plugin has {@link #getUpdateInfo() an update},
      * returns the {@link UpdateCenter.Plugin} object.
+     *
+     * @return
+     *      This method may return null — for example,
+     *      the user may have installed a plugin locally developed.
      */
     public UpdateCenter.Plugin getUpdateInfo() {
         UpdateCenter uc = Hudson.getInstance().getUpdateCenter();