std::str::FromStr should mention it gets used in str::parse

[frewsxcv]

On the surface, the FromStr seems like a pretty pointless trait, but it should maybe mention it gets used in str::parse so it seems slightly less pointless

https://doc.rust-lang.org/nightly/std/str/trait.FromStr.html

[sfackler]

👍

[steveklabnik]

part of #29375

[devonhollowood]

I can do this.