Document #[darling(supports(…))] shape attributes

[Jason Heeris]

There are attributes for enforcing particular shapes that are supported by Darling's attributes: #[darling(supports(…))] (and #[darling(ignore(…))]?). These are in core/src/options/shape.rs with an example here.

It looks like the docs are already there, but need to be surfaced and perhaps expanded on.