message: Add a comment about Decimal*::sqrt algorithm
diff: @@ -79,6 +79,10 @@ impl Decimal256 {
  // Algorithm described in https://hackmd.io/@webmaster128/SJThlukj_
  // We start with the highest precision possible and lower it until
  // there's no overflow.
  + //
  + // TODO: This could be made more efficient once log10 is in:
  + // https://github.com/rust-lang/rust/issues/70887
  + // The max precision is something like `18 - log10(self.0) / 2`.
  (0..=18)
  .rev()
  .find_map(|i| self.sqrt_with_precision(i))
  