message: Add docstrings to CleanOptions
diff: @@ -10,8 +10,11 @@ use ops::{self, BuildConfig, Context, Kind, Unit};
  pub struct CleanOptions<'a> {
  pub config: &'a Config,
  + /// A list of packages to clean. If empty, everything is cleaned.
  pub spec: Vec<String>,
  + /// The target arch triple to clean, or None for the host arch
  pub target: Option<String>,
  + /// Whether to clean the release directory
  pub release: bool,
  }
  