Warn against using non-uniform scale for 3D physics

Using non-uniform scale is known to cause many issues.
Add warnings to the editor and to the class reference.
Also remove the warning from SoftBody3D since it is
not relevant there: it simulates in global space.

doc/classes/RigidBody3D.xml

@@ -8,6 +8,7 @@
 		You can switch the body's behavior using [member lock_rotation], [member freeze], and [member freeze_mode].
 		[b]Note:[/b] Don't change a RigidBody3D's position every frame or very often. Sporadic changes work fine, but physics runs at a different granularity (fixed Hz) than usual rendering (process callback) and maybe even in a separate thread, so changing this from a process loop may result in strange behavior. If you need to directly affect the body's state, use [method _integrate_forces], which allows you to directly access the physics state.
 		If you need to override the default physics behavior, you can write a custom force integration function. See [member custom_integrator].
+		[b]Warning:[/b] With a non-uniform scale this node will probably not function as expected. Please make sure to keep its scale uniform (i.e. the same on all axes), and change the size(s) of its collision shape(s) instead.
 	</description>
 	<tutorials>
 		<link title="Physics introduction">$DOCS_URL/tutorials/physics/physics_introduction.html</link>