Add instructions for configuring CheckBox as a radio button

doc/classes/BaseButton.xml

@@ -49,6 +49,7 @@
 		</member>
 		<member name="button_group" type="ButtonGroup" setter="set_button_group" getter="get_button_group">
 			The [ButtonGroup] associated with the button. Not to be confused with node groups.
+			[b]Note:[/b] The button will be configured as a radio button if a [ButtonGroup] is assigned to it.
 		</member>
 		<member name="button_mask" type="int" setter="set_button_mask" getter="get_button_mask" enum="MouseButtonMask" default="1">
 			Binary mask to choose which mouse buttons this button will respond to.

doc/classes/ButtonGroup.xml

@@ -4,7 +4,7 @@
 		Group of Buttons.
 	</brief_description>
 	<description>
-		Group of [BaseButton]. The members of this group are treated like radio buttons in the sense that only one button can be pressed at the same time.
+		Group of [BaseButton]. The members of this group are treated like radio buttons in the sense that only one button can be pressed at the same time. Some types of buttons (such as [CheckBox]) may have a special appearance for this state.
 		Every member of the ButtonGroup should have [member BaseButton.toggle_mode] set to [code]true[/code].
 	</description>
 	<tutorials>

doc/classes/CheckBox.xml

@@ -6,6 +6,7 @@
 	<description>
 		A checkbox allows the user to make a binary choice (choosing only one of two possible options). It's similar to [CheckButton] in functionality, but it has a different appearance. To follow established UX patterns, it's recommended to use CheckBox when toggling it has [b]no[/b] immediate effect on something. For example, it could be used when toggling it will only do something once a confirmation button is pressed.
 		See also [BaseButton] which contains common properties and methods associated with this node.
+		[b]Note:[/b] CheckBox changes its appearance when it's configured as a radio button. See various [code]radio_*[/code] theme properties. To configure CheckBox to act as a radio button, use [member BaseButton.button_group] and [ButtonGroup].
 	</description>
 	<tutorials>
 	</tutorials>