Document the BaseMaterial3D billboard mode not being suited for VR

See #41567.

(cherry picked from commit ecc47ce2358c4ed63a22c025885b7a9ba79db215)

doc/classes/SpatialMaterial.xml

@@ -252,6 +252,7 @@
 		</member>
 		<member name="params_billboard_mode" type="int" setter="set_billboard_mode" getter="get_billboard_mode" enum="SpatialMaterial.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="params_blend_mode" type="int" setter="set_blend_mode" getter="get_blend_mode" enum="SpatialMaterial.BlendMode" default="0">
 			The material's blend mode.