Add some links and minor explanatory comments to `std::fmt` I thought the documentation for the `#` flag could do with a link to the explanation of the `?xXbo` flags, because at that point they haven't been explained yet and it's a bit confusing. I also added that the `0` flag overrides the fill character and alignment flag, here's a [Rust Playgrond](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0d580b7b78b8a2d8c08a2fc7a936ef17) that shows what I mean.