Add width reference documentation for radio set

docs/widgets/radioset.md

@@ -0,0 +1,37 @@
+# RadioSet
+
+A container widget that groups [`RadioButton`](./radiobutton.md)s together.
+
+- [ ] Focusable
+- [x] Container
+
+## Example
+
+The example below shows two radio sets, one built using a collection of
+[radio buttons](./radiobutton.md), the other a collection of simple strings.
+
+=== "Output"
+
+    ```{.textual path="docs/examples/widgets/radio_set.py"}
+    ```
+
+=== "radio_set.py"
+
+    ```python
+    --8<-- "docs/examples/widgets/radio_set.py"
+    ```
+
+=== "radio_set.css"
+
+    ```sass
+    --8<-- "docs/examples/widgets/radio_set.css"
+    ```
+
+## Messages
+
+### ::: textual.widgets.RadioSet.Changed
+
+## See Also
+
+- [RadioSet](../api/radioset.md) code reference
+- [RadioButton](./radiobutton.md)