message: fix docs to drop reverted change
diff: Experimental Cargo features are only available on the nightly channel. You
  typically use one of the `-Z` flags to enable them. Run `cargo -Z help` to
  -see a list of flags available. All Z flags accept either dashes or underscores
  -as separators.
  +see a list of flags available.
  `-Z unstable-options` is a generic flag for enabling other unstable
  command-line flags. Options requiring this will be called out below.
  