Consider validating that docs can build as on docs.rs in CI

[alice-i-cecile]

I added the doc_auto_cfg feature to the same set of crates. It highlights which items are specific to feature or target. It is documented here, and only runs on docs.rs. Might be worth considering using https://github.com/dtolnay/cargo-docs-rs on CI.

Originally posted by @yrns in #12366 (comment)