tor-rtmock: Avoid confusing "concurrently" word

    ///

    /// `call` will be run on a separate thread, called a "Subthread".

    ///

    /// But it will **not run concurrently** with the executor,

    /// But it will **not run simultaneously** with the executor,

    /// nor with other Subthreads.

    /// So Subthreads are somewhat like coroutines.

    ///

    ///    [`progress_until_stalled`](MockExecutor::progress_until_stalled)

    ///    and the facilities which use it, such as

    ///    [`MockRuntime::advance_until_stalled`](crate::MockRuntime::advance_until_stalled).)

    ///  * Subthreads never run concurrently -

    ///  * Subthreads never run in parallel -

    ///    they only run as scheduled deterministically by the `MockExecutor`.

    ///    So using Subthreads eliminates a source of test nonndeterminism.

    ///    (Execution order is still varied due to explicitly varying the scheduling policy.)