Document VehicleBody3D and VehicleWheel3D limitations

These classes have dozens of open bugs and missing features
which may not be fixed anytime soon.

It's probably better to document it upfront at this point.

(cherry picked from commit 0493e7c106811e94ad8dbebb0b97c43b19e4023a)

doc/classes/VehicleBody.xml

@@ -6,6 +6,7 @@
 	<description>
 		This node implements all the physics logic needed to simulate a car. It is based on the raycast vehicle system commonly found in physics engines. You will need to add a [CollisionShape] for the main body of your vehicle and add [VehicleWheel] nodes for the wheels. You should also add a [MeshInstance] to this node for the 3D model of your car but this model should not include meshes for the wheels. You should control the vehicle by using the [member brake], [member engine_force], and [member steering] properties and not change the position or orientation of this node directly.
 		[b]Note:[/b] The origin point of your VehicleBody will determine the center of gravity of your vehicle so it is better to keep this low and move the [CollisionShape] and [MeshInstance] upwards.
+		[b]Note:[/b] This class has known issues and isn't designed to provide realistic 3D vehicle physics. If you want advanced vehicle physics, you will probably have to write your own physics integration using another [PhysicsBody] class.
 	</description>
 	<tutorials>
 	</tutorials>

doc/classes/VehicleWheel.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		This node needs to be used as a child node of [VehicleBody] and simulates the behavior of one of its wheels. This node also acts as a collider to detect if the wheel is touching a surface.
+		[b]Note:[/b] This class has known issues and isn't designed to provide realistic 3D vehicle physics. If you want advanced vehicle physics, you will probably have to write your own physics integration using another [PhysicsBody] class.
 	</description>
 	<tutorials>
 	</tutorials>