Safety comments in `MultiThreadedExecutor` are incorrect

[joseph-gio]

Both places where System::run_unsafe is called in MultiThreadedExecutor still have placeholder safety comments. They simply state that the safety invariants are upheld, without making any attempt to explain how they are upheld.

bevy/crates/bevy_ecs/src/schedule/executor/multi_threaded.rs

Line 443 in de10dce

// SAFETY: access is compatible

bevy/crates/bevy_ecs/src/schedule/executor/multi_threaded.rs

Line 612 in de10dce

// SAFETY: caller ensures system access is compatible