message: Document ToBigInt behaviour
  Adds documentation explaining why `ToBigInt` can return `None` in some cases, and when it can be ignored.
diff: @@ -2479,7 +2479,9 @@ impl<'de> serde::Deserialize<'de> for BigInt {
  }
  }
  -/// A generic trait for converting a value to a `BigInt`.
  +/// A generic trait for converting a value to a `BigInt`. This may return
  +/// `None` when converting from `f32` or `f64`, and will always succeed
  +/// when converting from any integer or unsigned primitive, or `BigUint`.
  pub trait ToBigInt {
  /// Converts the value of `self` to a `BigInt`.
  fn to_bigint(&self) -> Option<BigInt>;
  