Use `feature(doc_auto_cfg)` instead of `feature(doc_cfg)`

[divergentdave]

We currently use the #[doc_cfg] attribute to annotate documentation with conditional compilation expressions that gate different public items. We could instead use #[feature(doc_auto_cfg)] to automate this process. This would clean up extra attributes throughout the codebase, and catch any attributes we may have missed over time. However, if we have any cases where there are two definitions of an item, guarded by #[cfg(a)] and #[cfg(not(a))], we'd need to change that, because the automatically inferred conditions would be incorrect.