Appling the following command changing the margin but doesn't change the position of the DocumentRuler:
this.radRichTextBox.ActiveDocumentEditor.ChangeSectionPageMargin(new Padding((int)Unit.InchToDip(3.0)));
The result:
MethodInfo method = typeof(RadRichTextBoxRulerController).GetMethod("OnSeparatorsChanged", BindingFlags.NonPublic | BindingFlags.Instance);
method.Invoke(this.ruler.Controller, null);
<telerik:DocumentRuler x:Name="ruler">