message: doc(tree): mention special target `all` in CLI help
  Fixes
diff: @@ -26,7 +26,8 @@ pub fn cli() -> App {
  )
  .arg_features()
  .arg_target_triple(
  - "Filter dependencies matching the given target-triple (default host platform)",
  + "Filter dependencies matching the given target-triple (default host platform). \
  + Pass `all` to include all targets.",
  )
  .arg(
  Arg::with_name("no-dev-dependencies")
  