Call this when the user asks how passage timing, complexity, or
    boat speed are computed (e.g. "comment c'est calculé ?",
    "what assumptions does the model use?", "is tacking modelled?").

    The returned text covers: polar lookup, default efficiency 0.75,
    VMG / tacking correction, wave derate, single-pass timing,
    compare-windows mode semantics, regional defaults, wind against
    current, short-period chop, what is intentionally not modelled, and
    the data attributions.