Add some comments documenting locations where
PhysicsServer3D::soft_body_set_mesh() can deadlock.
godotengine/godot-proposals#12670 has a proposal for some alternate
thread-safe soft body APIs. In the meantime it seems worth at least
documenting some of the current pitfalls in the code.
If the module is enabled (default), 3D physics works as it did before.
If the module is disabled and no other 3D physics server is registered
(via a module or GDExtension), then we fall back to a dummy
implementation which effectively disables 3D physics functionality (and
a warning is printed).
The dummy 3D physics server can also be selected explicitly, in which
case no warning is printed.
