FromStr::from_str method doc comment is inaccurate

http://doc.rust-lang.org/1.0.0-beta.4/std/str/trait.FromStr.html#tymethod.from_str

> Parses a string s to return an optional value of this type. If the string is ill-formatted, the None is returned.

This description is probably leftover from when `from_str` returned an `Option`
