Document private items in CI?

[joshlf]

If we passed --document-private-items to cargo doc in CI, it'd ensure that private items' documentation compiles successfully. This isn't that important, but it makes it easier to promote private items to public ones since we ensure from the beginning that the documentation is valid.

By a similar token, we could use --document-hidden-items. This affects #[doc(hidden)] items, which are more likely to be promoted to un-hidden than private items are to be promoted to public. However, --document-hidden-items is unstable and is not recognized by cargo doc, so we could only use it on nightly, and we'd need to pass it as RUSTDOCFLAGS="-Z unstable-options --document-hidden-items". Stability is not that big of a deal because, if the option were ever removed, we could easily update our CI to no longer make use of it.