message: document why it is safe to call unwrap in the `Entropy::to_mnemonics`
  relates to
diff: @@ -332,7 +332,8 @@ impl Entropy {
  // only store up to the value 2047
  words.push(MnemonicIndex::new(n).unwrap());
  }
  -
  + // by design, it is safe to call unwrap here as
  + // the mnemonic length has been validated by construction.
  Mnemonics::from_mnemonics(words).unwrap()
  }
  }
  