Document the BaseMaterial3D billboard mode not being suited for VR

See #41567.

doc/classes/BaseMaterial3D.xml

@@ -122,6 +122,7 @@
 		</member>
 		<member name="billboard_mode" type="int" setter="set_billboard_mode" getter="get_billboard_mode" enum="BaseMaterial3D.BillboardMode" default="0">
 			Controls how the object faces the camera. See [enum BillboardMode].
+			[b]Note:[/b] Billboard mode is not suitable for VR because the left-right vector of the camera is not horizontal when the screen is attached to your head instead of on the table. See [url=https://github.com/godotengine/godot/issues/41567]GitHub issue #41567[/url] for details.
 		</member>
 		<member name="blend_mode" type="int" setter="set_blend_mode" getter="get_blend_mode" enum="BaseMaterial3D.BlendMode" default="0">
 			The material's blend mode.