message: Document the purpose of __add2
diff: @@ -122,7 +122,12 @@ pub fn rem_digit(a: &BigUint, b: BigDigit) -> BigDigit {
  rem
  }
  -// Only for the Add impl:
  +/// Two argument addition of raw slices, `a += b`, returning the carry.
  +///
  +/// This is used when the data `Vec` might need to resize to push a non-zero carry, so we perform
  +/// the addition first hoping that it will fit.
  +///
  +/// The caller _must_ ensure that `a` is at least as long as `b`.
  #[inline]
  pub fn __add2(a: &mut [BigDigit], b: &[BigDigit]) -> BigDigit {
  debug_assert!(a.len() >= b.len());
  