cargo doc output not very useful currently. Fix!

[rrnewton]

Good packages on cargo have nice, self-explanatory documentation (just like Hackage!). For instance, here's crossbeam:

https://docs.rs/crossbeam/0.7.1/crossbeam/

If I run cargo doc under systrace, the result is not very useful. There is no top-level explanation, and drilling down into individual modules just gives mysterious lists of symbol names with no explanation.