Added language to provide an explanation of the use of the 'order' ke…

…y when registering a back-end menu item (#306)

Credit to @ibanks

plugin-registration.md

@@ -231,7 +231,7 @@ Plugins can extend the back-end navigation menus by overriding the `registerNavi
         ];
     }
 
-When you register the back-end navigation you can use [localization strings](localization) for the `label` values. Back-end navigation can also be controlled by the `permissions` values and correspond to defined [back-end user permissions](../backend/users).
+When you register the back-end navigation you can use [localization strings](localization) for the `label` values. Back-end navigation can also be controlled by the `permissions` values and correspond to defined [back-end user permissions](../backend/users). The order in which the back-end navigation appears on the overall navigation menu items, is controlled by the `order` value. Higher numbers mean that the item will appear later on in the order of menu items while lower numbers mean that it will appear earlier on.
 
 To make the sub-menu items visible, you may [set the navigation context](../backend/controllers-ajax#navigation-context) in the back-end controller using the `BackendMenu::setContext` method. This will make the parent menu item active and display the children in the side menu.