Tweak CanvasGroup description to be more explicit about its purpose

doc/classes/CanvasGroup.xml

@@ -4,8 +4,8 @@
 		Merges several 2D nodes into a single draw operation.
 	</brief_description>
 	<description>
-		Child [CanvasItem] nodes of a [CanvasGroup] are drawn as a single object. It allows to e.g. draw overlapping translucent 2D nodes without blending (set [member CanvasItem.self_modulate] property of [CanvasGroup] to achieve this effect).
-		[b]Note:[/b] The [CanvasGroup] uses a custom shader to read from the backbuffer to draw its children. Assigning a [Material] to the [CanvasGroup] overrides the builtin shader. To duplicate the behavior of the builtin shader in a custom [Shader] use the following:
+		Child [CanvasItem] nodes of a [CanvasGroup] are drawn as a single object. It allows to e.g. draw overlapping translucent 2D nodes without causing the overlapping sections to be more opaque than intended (set the [member CanvasItem.self_modulate] property on the [CanvasGroup] to achieve this effect).
+		[b]Note:[/b] The [CanvasGroup] uses a custom shader to read from the backbuffer to draw its children. Assigning a [Material] to the [CanvasGroup] overrides the built-in shader. To duplicate the behavior of the built-in shader in a custom [Shader], use the following:
 		[codeblock]
 		shader_type canvas_item;
 		render_mode unshaded;