Add more cross-links throughout documentation

[emlun]

Hopefully the title says it all. 🙂 I saw that intra-rustdoc links are used in some cases but not all, so I "upgraded" all the ones I could find where it seems appropriate. I did not add links to references to the containing type, for example in "Attempt to create a Date from the year, month, and day." on Date::from_ymd, since that seems a bit redundant. I also did not rewrap any lines as I'm not sure about the convention on that here, but I'd be happy to do so if desired.

[jhpratt]

I'll take a look at this tomorrow, but there's no reason to not do this. With regard to comment wrapping, I generally cap the line length to 80 (including indentation). Unfortunately rustfmt breaks the markdown table in the crate-level documentation, otherwise I'd hand control over to that.