Updated the navigation doc regarding show_menu_below_id with soft roots

Author: db654

docs/basic_reference/navigation.rst

@@ -96,6 +96,9 @@ You can give it the same optional parameters as :ttag:`show_menu`::
         {% show_menu_below_id "meta" 0 100 100 100 "myapp/menu.html" %}
     </ul>
 
+Unlike :ttag:`show_menu`, however, soft roots will not affect the menu when 
+using :ttag:`show_menu_below_id`.
+
 *************
 show_sub_menu
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