Auto merge of #112152 - jyn514:doc-msg, r=clubby789

Fix the progress message for `x doc rustc`

This makes it more clear that we're using stage 0 *to document* rustc, not that we're documenting stage0 rustc itself.

It also fixes a bug in `msg_sysroot_tool` that would print `Docing`, and removes the `Debug` impl for `Kind` to make sure it doesn't happen again.

Before:
```
Documenting stage0 compiler {rustc-main} (aarch64-apple-darwin)
```

After:
```
Documenting compiler {rustc-main} (stage0 -> stage1, aarch64-apple-darwin)
```

thanks `@BoxyUwU` for catching this!