docs: rust: Improve wording about adding Cargo.lock to src (#340287)

}

```

Note that setting `cargoLock.lockFile` or `cargoLock.lockFileContents`

doesn't add a `Cargo.lock` to your `src`, and a `Cargo.lock` is still

required to build a rust package. A simple fix is to use:

If the upstream source repository lacks a `Cargo.lock` file, you must add one

to `src`, as it is essential for building a Rust package. Setting

`cargoLock.lockFile` or `cargoLock.lockFileContents` will not automatically add

a `Cargo.lock` file to `src`. A straightforward solution is to use:

```nix

{