message: Remove rustdoc warnings for core::features
  Link `CliUnstable::add` just to `CliUnstable` instead.
diff: //!
  //! 1. Add the option to the [`CliUnstable`] struct below. Flags can take an
  //! optional value if you want.
  -//! 2. Update the [`CliUnstable::add`] function to parse the flag.
  +//! 2. Update the [`CliUnstable::add`][CliUnstable] function to parse the flag.
  //! 3. Wherever the new functionality is implemented, call
  //! [`Config::cli_unstable`][crate::util::config::Config::cli_unstable] to
  //! get an instance of `CliUnstable` and check if the option has been
  //! macro below.
  //! 2. `-Z unstable-options`: Find the call to `fail_if_stable_opt` and
  //! remove it. Be sure to update the man pages if necessary.
  -//! 3. `-Z` flag: Change the parsing code in [`CliUnstable::add`] to call
  -//! `stabilized_warn` or `stabilized_err` and remove the field from
  +//! 3. `-Z` flag: Change the parsing code in [`CliUnstable::add`][CliUnstable]
  +//! to call `stabilized_warn` or `stabilized_err` and remove the field from
  //! `CliUnstable. Remove the `(unstable)` note in the clap help text if
  //! necessary.
  //! 2. Remove `masquerade_as_nightly_cargo` from any tests, and remove
  