Document that the Godot physics engines aren't deterministic

**Your Godot version:** 3.3.3

**Issue description:** None of the physics engines provided by Godot are deterministic, regardless if you use RigidBody or KinematicBody. This should be documented somewhere because it has implications on networking and replay systems. For instance, you need to store players' transforms in replays instead of players' inputs to avoid random desyncs that would happen otherwise.

See also https://godotengine.org/qa/114615/is-the-godot-engine-reliable-for-counting-frames.

**URL to the documentation page (if already existing):** Not sure, any suggestions?