message: Fix some typos in docs and error messages
diff: //! the `Manifest::feature_gate` function, but otherwise you may wish to
  //! place the feature gate elsewhere in Cargo.
  //!
  -//! 3. Do actually perform the feature gate, you'll want to have code that looks
  +//! 3. To actually perform the feature gate, you'll want to have code that looks
  //! like:
  //!
  //! ```rust,ignore
  @@ -148,7 +148,7 @@ impl Features {
  };
  if *slot {
  - bail!("the cargo feature `{}` has already bene activated", feature);
  + bail!("the cargo feature `{}` has already been activated", feature);
  }
  match status {
  @@ -200,7 +200,7 @@ impl Features {
  }
  }
  -/// A parsed represetnation of all unstable flags that Cargo accepts.
  +/// A parsed representation of all unstable flags that Cargo accepts.
  ///
  /// Cargo, like `rustc`, accepts a suite of `-Z` flags which are intended for
  /// gating unstable functionality to Cargo. These flags are only available on
  