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!