Update docstring for RadioGroup class

Clarified the docstring to specify usage within Label.

Author: Sacul0457

discord/ui/radio.py

@@ -54,7 +54,7 @@
 
 
 class RadioGroup(Item[V]):
-    """Represents a radio group component within a modal.
+    """Represents a radio group component within a modal that can only be used in :class:`Label`.
 
     .. versionadded:: 2.7