Documentation (#6200)

* vim backup files to be ignored

* added tip about using document$ observable

docs/customization.md

@@ -54,6 +54,29 @@ extra_javascript:
   - javascripts/extra.js
 ```
 
+!!! tip "Make sure the page is fully loaded"
+
+    It is likely that you will want to run your JavaScript code only
+    once the page has been fully loaded by the browser. This means
+    installing a callback function subscribing to events on the
+    `document$` observable exported by Material for MkDocs.
+    Using the `document$` observable is particularly important if you
+    are using [instant loading] since it will not result in a page
+    refresh in the browser - but subscribers on the observable will be
+    notified.
+
+    ``` javascript
+    document$.subscribe(function() {
+      console.log("Page loaded.") // or whatever it is you want to do
+    })
+    ```
+
+    `document$` is an [RxJS Observable] and you can call the `subscribe()`
+    method any number of times to attach different functionality.
+
+    [instant loading]: setup/setting-up-navigation.md/#instant-loading
+    [RxJS Observable]: https://rxjs.dev/api/index/class/Observable
+
 ## Extending the theme
 
 If you want to alter the HTML source (e.g. add or remove some parts), you can