Document all feature, and show feature requirements

[michaelkirk]

• [x] I agree to follow the project's code of conduct.
• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

• [x] I ran cargo fmt

This is similar to what we do in geojson and elsewhere. Though I just learned about doc_auto_cfg which makes this way easier by automatically annotating any required items, rather than needing to try to sprinkle a bunch of calls all over like:

#[cfg_attr(docsrs, doc(cfg(feature = "geo-types")))]
mod my_geo_types_module;

e.g.

[michaelkirk]

bors r=frewsxcv

[bors[bot]]

Build succeeded:

• wkt