Document known performance issues with Sprite3D

See https://github.com/godotengine/godot/issues/20855.

doc/classes/Sprite3D.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		A node that displays a 2D texture in a 3D environment. The texture displayed can be a region from a larger atlas texture, or a frame from a sprite sheet animation.
+		[b]Note:[/b] There are [url=https://github.com/godotengine/godot/issues/20855]known performance issues[/url] when using [Sprite3D]. Consider using a [MeshInstance] with a [QuadMesh] as the mesh instead. You can still have billboarding by enabling billboard properties in the QuadMesh's [StandardMaterial3D].
 	</description>
 	<tutorials>
 	</tutorials>