`algorithms/mod`: document algorithms

[github-actions[bot]]

On 2023-09-30 @prestwich wrote in d705f65 “Merge pull request #322 from DaniPopes/more-clippies”:

document algorithms

//! ⚠️ Collection of bignum algorithms.
//!
//! **Warning.** Most functions in this module are currently not considered part
//! of the stable API and may be changed or removed in future minor releases.

#![allow(missing_docs)] // TODO: document algorithms

mod add;
pub mod div;
mod gcd;
mod mul;

From src/algorithms/mod.rs:6