Document some deadlocks in the physics server code

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.

modules/godot_physics_3d/godot_soft_body_3d.cpp

@@ -136,6 +136,10 @@ void GodotSoftBody3D::set_mesh(RID p_mesh) {
 		return;
 	}
 
+	// TODO: calling RenderingServer::mesh_surface_get_arrays() from the physics thread
+	// is not safe and can deadlock when physics/3d/run_on_separate_thread is enabled.
+	// This method blocks on the main thread to return data, but the main thread may be
+	// blocked waiting on us in PhysicsServer3D::sync().
 	Array arrays = RenderingServer::get_singleton()->mesh_surface_get_arrays(soft_mesh, 0);
 	ERR_FAIL_COND(arrays.is_empty());
 

modules/jolt_physics/objects/jolt_soft_body_3d.cpp

@@ -128,6 +128,10 @@ bool JoltSoftBody3D::_ref_shared_data() {
 	if (iter_shared_data == mesh_to_shared.end()) {
 		RenderingServer *rendering = RenderingServer::get_singleton();
 
+		// TODO: calling RenderingServer::mesh_surface_get_arrays() from the physics thread
+		// is not safe and can deadlock when physics/3d/run_on_separate_thread is enabled.
+		// This method blocks on the main thread to return data, but the main thread may be
+		// blocked waiting on us in PhysicsServer3D::sync().
 		const Array mesh_data = rendering->mesh_surface_get_arrays(mesh, 0);
 		ERR_FAIL_COND_V(mesh_data.is_empty(), false);