Correctly document the CRUBIT_THREAD_SAFE annotation.

[copybara-service]

Correctly document the CRUBIT_THREAD_SAFE annotation.

The issue is that if the fields are not marked mutable, then it is UB to store it in a const object and pass it to Rust.

You might say, "why store it as a const object?" but C++ programmers sometimes store anything as const that they can, and if their only use of it ends up being passing it to Rust, and Rust only requires a const reference... oops!

struct CRUBIT_THREAD_SAFE MyType {
  void Mutate() { mu_.Lock(); x_++; mu_.Unlock();}
  int x_ = 0;  // not mutable
  mutable Mutex mu_;
};

pub fn MyRustFunction(x: &MyType) {
  x.Mutate();
}

const MyType x;
MyRustFunction(x); // UB