message: Document -Z extra-link-arg.
diff: @@ -36,6 +36,7 @@ pub fn main(config: &mut Config) -> CliResult {
  Available unstable (nightly-only) flags:
  -Z avoid-dev-deps -- Avoid installing dev-dependencies if possible
  + -Z extra-link-arg -- Allow `cargo:rustc-link-arg` in build scripts
  -Z minimal-versions -- Install minimal dependency versions instead of maximum
  -Z no-index-update -- Do not update the registry, avoids a network request for benchmarking
  -Z unstable-options -- Allow the usage of unstable options
  