Add doc links between `{integer}::from_str_radix` and `from_str`
When parsing base-10 numbers, it's easy to miss `<Self as FromStr>::from_str` and `str::parse` as potential alternatives to `from_str_radix`.
- A similar suggestion is given by https://rust-lang.github.io/rust-clippy/master/index.html#from_str_radix_10