supported more complex relative path

core/src/main/java/hudson/RelativePath.java

@@ -14,8 +14,23 @@ import static java.lang.annotation.RetentionPolicy.*;
  * nearby parameters that belong to different parents.
  *
  * <p>
- * Currently, "..", "../..", etc. are supported to indicate
- * parameters that belong to the ancestors.
+ * ".." refers
+ * to values in the parent object, and "foo" refers to the child
+ * object of the current object named "foo". They can be combined
+ * with '/' like path, such as "../foo/bar", "../..", and etc.
+ *
+ * <p>
+ * A good way to think about this is the file system structure.
+ * {@code @RelativePath} is like the dirname, and {@code QueryParameter}
+ * is like the basename. Together they form a relative path.
+ * And because of the structured form submissions,
+ * form elements are organized in a tree structure of JSON objects,
+ * which is akin to directories and files.
+ *
+ * <p>
+ * The relative path then points from the current input element
+ * (for which you are doing form validation, for example) to the target
+ * input element that you want to obtain the value.
  *
  * @author Kohsuke Kawaguchi
  * @since 1.376