Suppress unused argument warnings in unavoidable cases

It would be nice if Eclipse were smart enough to ignore unused
argument warnings on @UiHandler and @Option methods, but alas it does
not. Suppressing these is ugly, but based on all the cleanup in the
previous commit, the benefit outweighs the cost.

Change-Id: I743917787ffb53be034eee3af2c525652fc18cb0

gerrit-server/src/main/java/com/google/gerrit/rules/StoredValue.java

@@ -84,7 +84,12 @@ public class StoredValue<T> {
     env.set(this, obj);
   }
 
-  /** Creates a value to store, returns null by default. */
+  /**
+   * Creates a value to store, returns null by default.
+   *
+   * @param engine Prolog engine.
+   * @return new value.
+   */
   protected T createValue(Prolog engine) {
     return null;
   }