Document the Radio widget

docs/dev/reference/widgets/radio.md

@@ -1,6 +1,76 @@
 ---
 title: Radio
-description: Renders a single or multiple radio buttons (not yet documented)
+description: Renders a radio button selection.
 ---
 
-{{< widget-notice >}}
+This widget renders a radio button selection.
+
+![Radio buttons]({{% asset "images/dev/reference/widgets/radio.png" %}}?classes=shadow)
+
+## Options
+
+This table only shows the options relevant to the core functionality of this widget. See the DCA reference for a
+[full field reference]({{% relref "fields" %}}).
+
+| Key   | Value | Description
+| ----- | ----- | --------------- |
+| `inputType` | `select` | |
+| `options` | `array` | An options array (use in combination with `eval.multiple`) |
+| `options_callback` | `function\|callable` | A callback function that returns the options callback or an array  (use in combination with `eval.multiple`). You may define an anonymous function, but you should consider [registering them via service tagging]({{% relref "reference/dca#registering-callbacks" %}}). |
+| `reference` | `array` | Reference an array that will be used to translate the options. Contao will automatically match the options and reference array by key. |
+| `foreignKey` | `string` | Reference another table to generate options from. |
+| `eval.includeBlankOption` | true/false (default) `bool` | Includes a blank option (useful in conjunction with `mandatory` fields) |
+| `eval.blankOptionLabel` | `string` (default `-`) | The label of the blank option |
+| `eval.disabled` | true/false (default) `bool` | Disables the input field |
+
+## Examples
+
+{{< tabs groupid="radio-widget-examples" style="code" >}}
+
+{{% tab title="Simple radio buttons" %}}
+
+If you simply want to select an option from a fixed set.
+
+```php
+// …
+'example' => [
+    'label' => ['Example', 'Example radio buttons.'],
+    'inputType' => 'radio',
+    'options' => [
+        'lorem' => 'Lorem',
+        'ipsum' => 'Ipsum',
+        'dolor' => 'Dolor',
+    ],
+    'sql' => [
+        'type' => 'string',
+        'length' => 8, // Must be large enough to store all possible values
+        'default' => 'lorem',
+    ],
+],
+// …
+```
+
+{{% /tab %}}
+
+{{% tab title="Options from a table" %}}
+
+You can generate an options array from another table with the `foreignKey` property. 
+
+```php
+// …
+'example' => [
+    'label' => ['Example', 'Example radio buttons.'],
+    'inputType' => 'radio',
+    'foreignKey' => 'tl_user.name', // Will use `name` as label, and the user `id` as value
+    'sql' => [
+        'type' => 'integer',
+        'unsigned' => true,
+        'default' => 0,
+    ],
+],
+// …
+```
+
+{{% /tab %}}
+
+{{< /tabs >}}