DOC Document general loazy-loading of DOM elements

en/02_Developer_Guides/15_Customising_the_Admin_Interface/02_CMS_Architecture.md

@@ -772,6 +772,26 @@ The URL endpoints `$AdminURL('mytabs/tab1')` and `$AdminURL('mytabs/tab2')`
 should return HTML fragments suitable for inserting into the content area,
 through the `PjaxResponseNegotiator` class (see above).
 
+### Lazy loading fields on tab activation
+
+When a tab is not lazy loaded via ajax, it might still be necessary to
+delay some work (eg: when doing http requests) until the tab is activated. This is how, for example, the
+[`GridFieldLazyLoader`](api:SilverStripe\Forms\GridField\GridFieldLazyLoader) works.
+
+In order to open up the same kind of features to other fields, a custom event is fired on all nodes with the `lazy-loadable` class inside the activated tab panel.
+They will receive a `lazyload` event that can be listened to in the following way (you will have to implement your own logic for "loading" the content):
+
+```js
+jQuery('.my-element.lazy-loadable').entwine({
+    // Use onmatch so we apply the event handler as soon as the element enters the DOM
+    onmatch(e) {
+        this.one('lazyload', () => {
+            // Some init code here
+        });
+    },
+});
+```
+
 ## Related
 
 - [Howto: Extend the CMS Interface](/developer_guides/customising_the_admin_interface/how_tos/extend_cms_interface)