Docs: Write a page for QUnit.config.modules

Used by HTML Reporter, and by plugins such as `steal-qunit`.

Ref #1118

docs/config/current.md

@@ -13,7 +13,7 @@ Internal object representing the currently running test.
 <table>
 <tr>
   <th>type</th>
-  <td markdown="span">`object`</td>
+  <td markdown="span">`object` (read-only)</td>
 </tr>
 </table>
 

docs/config/modules.md

@@ -0,0 +1,39 @@
+---
+layout: page-api
+title: QUnit.config.modules
+excerpt: List of defined test modules.
+groups:
+  - config
+  - extension
+version_added: "1.16.0"
+---
+
+List of defined test modules.
+
+<table>
+<tr>
+  <th>type</th>
+  <td markdown="span">`Array<Module>` (read-only)</td>
+</tr>
+</table>
+
+This property is exposed under `QUnit.config` for use by plugins and other integrations. It returns an array of internal `Module` objects, one for each call to [`QUnit.module()`](../QUnit/module.md).
+
+Before accessing this property, wait for the [`QUnit.on('runStart')`](../callbacks/QUnit.on.md#the-runstart-event) event, or use a [`QUnit.begin()`](../callbacks/QUnit.begin.md) callback.
+
+### Module object
+
+The following properties are considered publicly supported:
+
+| property | description |
+|-----------|-------------|
+| `name` (string) | Module name, as passed to [`QUnit.module()`](../QUnit/module.md).
+| `moduleId` (string) | Hashed identifier, for the [QUnit.config.moduleId](./moduleId.md) filter.
+
+## Example
+
+```js
+QUnit.on('runStart', () => {
+  console.log(QUnit.config.modules.map(mod => mod.name));
+});
+```