Mention in safety comment that Parcel can't be Sync.

/// Safety: This type guarantees that it owns the AParcel and that all access to

/// the AParcel happens through the Parcel, so it is ok to send across threads.

///

/// It would not be okay to implement Sync, because that would allow you to call

/// the reading methods from several threads in parallel, which would be a data

/// race on the cursor position inside the AParcel.

unsafe impl Send for Parcel {}

/// Container for a message (data and object references) that can be sent