Documentation examples styles

[srrrse]

Proposing the following norms for documentation (which mostly matches what we've done so far):

1. Methods only include statements, function signatures are hidden.
2. Type and module level examples include function signatures, but no main function.
3. Main function in examples, lib.rs, and README.
4. Nothing is shown as visibly imported unless its used in the visible code.
5. No ::std visible in docs; if its used in the shown part of the doc, its imported.

[Yosh]

These norms sound good to me!

[Yosh]

Closed by #37 🎉