NetDir::pick_n_relays and pick_n_weighted: clarify/fix docs re zero-weight items

    /// returned twice. Therefore, the resulting vector may be smaller

    /// than `n` if we happen to have fewer than `n` appropriate relays.

    ///

    /// This function returns an empty vector if (and only if) there

    /// are no relays with nonzero weight.

    /// If *all* items have zero-weight, then up to `n` will be chosen randomly

    /// and returned. Otherwise, never returns items with zero-weight.

    #[allow(clippy::cognitive_complexity)] // all due to tracing crate.

    fn pick_n_weighted<R, T>(rng: &mut R, n: usize, weighted_items: &[(T, u64)]) -> Vec<T>

    where

    /// returned twice. Therefore, the resulting vector may be smaller

    /// than `n` if we happen to have fewer than `n` appropriate relays.

    ///

    /// This function returns an empty vector if (and only if) there

    /// are no relays with nonzero weight where `usable` returned

    /// true.

    /// Relays with zero-weight will be chosen only if there are *no* usable

    /// relays with nonzero-weight.

    ///

    /// This function returns an empty vector if (and only if) there are no

    /// relays where `usable` returned true.

    #[allow(clippy::cognitive_complexity)] // all due to tracing crate.

    pub fn pick_n_relays<'a, R, P>(

        &'a self,