Augment RuntimeClassInitialization#initializeAtBuildTime javadoc

Document that using `""` as the package name results in all packages and
classes being registered for build time initialization.

sdk/src/org.graalvm.nativeimage/src/org/graalvm/nativeimage/hosted/RuntimeClassInitialization.java

@@ -150,6 +150,8 @@ public static void initializeAtRunTime(String... packages) {
     /**
      * Registers all classes in provided packages as eagerly initialized during image-build time.
      * <p>
+     * Passing {@code ""} as package, registers all packages and classes for initialization at build time.
+     * <p>
      * All static initializers of {@code classes} will be executed during image-build time and
      * static fields that are assigned values will be available at runtime. {@code static final}
      * fields will be considered as constant.