Rework reference-style documentation

[TedDriggs]

We have two parallel problems:

1. It's hard for crate users to find all the options for builder attributes and their effects
2. It's hard for crate maintainers to make sure new options are findable and documented

These feed on each other: As primary maintainer, I don't know where best to document #228, and therefore no crate user will know where to read about how to control the creation of constructors or build functions.

We can separate documentation into some areas:

1. High-level introduction
2. Use-case examples
3. Attribute-level references

This issue is going to focus on item #3: Reference-style documentation.

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Thanks to darling, our attributes are actually fairly understandable in darling_opts.rs - it's easy to see the root options item, and then each of the fields from there, and to recurse down to see what each one does. Unfortunately, those aren't exposed by rustdoc because those structs aren't public.

Question 1: How best to surface these? Big reference in README.md? Crate-level docs in the readme block?