Adjusted Loader widget documentation (#5230)

* Adjusted loader widget documentation

* Adjusted the loader documentation

Co-Authored-By: Yaroslav Rogoza <[email protected]>

* Adjusted the loader widget documentation

* Fix the issue with trailing spaces

* Added the loader widget result

Author: serhiyzhovnir

common/images/widget/loader-widget-result.png

@@ -6,13 +6,37 @@ title: Loader widget
 
 ## Overview
 
-The Loader [widget](https://glossary.magento.com/widget) blocks page content (all content or a part of it). Its intended use is blocking content when an Ajax request is being sent. But it can be initialized for non-Ajax tasks as well.
+The Loader [widget](https://glossary.magento.com/widget) blocks page content (all content or a part of it). Its intended use is blocking content when an Ajax request is being sent, but it can be initialized for non-Ajax tasks as well.
 
 The Loader widget source is [lib/web/mage/loader.js].
 
 ## Initialize the loader widget {#loader_init}
 
-The loader widget is initialized as described in [JavaScript initialization].
+For information about how to initialize a widget in a JS component or `.phtml` template, see the [Initialize JavaScript][] topic.
+
+The following example shows how to instantiate the Loader widget:
+
+```javascript
+$('#example').loader({
+    icon: 'path to icon'
+});
+```
+
+Where:
+
+- `#element` is the selector of the element for Loader is initialized.
+
+```html
+<script>
+require(['jquery', 'loader'], function ($) {
+    'use strict';
+
+    $('#example').loader({
+        icon: 'path to icon'
+    });
+});
+</script>
+```
 
 ## Options {#loader_options}
 
@@ -22,13 +46,15 @@ The loader widget has the following options:
 -   [texts](#l_texts)
 
 ### `icon` {#l_icon}
+
 The URL to the loader image. This image is displayed when the widget is active; that is, between the `ajaxSend` and `ajaxComplete` events.
 
 **Type**: String
 
 **Default value**: No image by default.
 
 ### `template` {#l_template}
+
 HTML wrapper for the output, or a DOM element selector.
 
 **Default value**:
@@ -81,6 +107,7 @@ Loader is subscribed to the following events:
 -   [processStop](#l_processStop)
 
 ### `processStart` {#l_processStart}
+
 Display the loader. Can be triggered on any page element.
 
 Start show loading:
@@ -89,6 +116,16 @@ Start show loading:
 $("body").trigger('processStart');
 ```
 
+The following is an example of adding a callback to `processStart` event.
+
+```javascript
+var callback = function () {
+    // do something before showing the loader
+};
+
+$("body").on('processStart', callback);
+```
+
 ### `processStop` {#l_processStop}
 Hide the loader. Can be triggered on any page element.
 
@@ -98,5 +135,39 @@ Stop show loading:
 $("body").trigger('processStop');
 ```
 
+The following is an example of adding a callback to a `processStop` event.
+
+```javascript
+var callback = function () {
+    // do something before hiding the loader
+};
+
+$("body").on('processStop', callback);
+```
+
+## Code Sample
+
+The following example shows the loader when a `processStart` event triggers for the `#loader-example` element.
+
+```html
+<div id="loader-example" data-mage-init='{"loader": { "icon": "{store URL}/static/{static version}/frontend/Magento/luma/en_US/images/loader-2.gif"}}'>
+    <h3>Block</h3>
+    Lorem ipsum dolor sit amet, consectetur adipisicing elit. Dignissimos itaque numquam placeat quam recusandae velit voluptas. Ad architecto asperiores eos eveniet id nostrum officiis saepe soluta totam voluptate! Debitis, quibusdam.
+</div>
+<script>
+require(['jquery', 'loader', 'domReady!'], function ($) {
+    'use strict';
+
+    $('#loader-example').trigger('processStart');
+});
+</script>
+```
+
+## Result
+
+The loader shows when a `processStart` event triggers for the `#loader-example` element.
+
+![Loader Widget Example]({{ site.baseurl }}/common/images/widget/loader-widget-result.png)
+
 [lib/web/mage/loader.js]: {{ site.mage2bloburl }}/{{ page.guide_version }}/lib/web/mage/loader.js
-[JavaScript initialization]: {{page.baseurl}}/javascript-dev-guide/javascript/js_init.html
+[Initialize JavaScript]: {{page.baseurl}}/javascript-dev-guide/javascript/js_init.html