Want mechanism for distinguishing "Rust doc" from "API doc"

At the moment, I believe the two are intermingled: If one wants to document a Rust type with rustdoc, that documentation is implicitly exported as part of the openapi spec. Similarly, if one wants to add openapi documentation, that must be done via Rust comments.

Although it may be a reasonable default for these two documentation targets to have the same source, it's reasonable to want:

• API docs not part of the Rust comments (i.e., they are applicable to the generated type), or
• Rust docs not part of the Open API spec (i.e., they reference other Rust types, or Rust-specific features)

Here's an example: https://github.com/oxidecomputer/omicron/pull/1348/files/b427eded0dfdc10b7c9461a38f110e9c620d0f5f#r918269331

It would be nice to have a mechanism to annotate structs and enums passed through dropshot with "OpenAPI" vs "Rustdoc" specific usage, and filter these before generating the open API spec.

Start by reading the issue and the referenced pull request discussion, then trace how Dropshot combines Rust comments with generated OpenAPI documentation. Define the annotation behavior for structs and enums, including how Rust-only and OpenAPI-only text is identified. Done means users can keep the two documentation targets separate when generating the OpenAPI specification.