circmgr: Document caveats about BuildCircuit estimator

#[derive(Copy, Clone, Debug, Eq, PartialEq)]

pub enum Action {

    /// Build a circuit of a given length.

    ///

    /// Note that you should not generally have to use

    /// this timeout outside of the circmgr code:

    /// The circmgr already computes and applies

    /// timeouts for creating and extending circuits.

    ///

    /// If you are using this timeout to estimate how long

    /// it will take somebody _else_ to build a circuit,

    /// remember that Tor clients will retry circuits when they fail,

    /// and so it may take longer than this for your peer to

    /// actually get a working circuit.

    BuildCircuit {

        /// The length of the circuit to construct.

        ///