Hide docs for core::unicode

[clarfonthey]

This module is perma-unstable and shouldn't show up in the public docs. If people want to see the docs for it, they can still run RUSTDOCFLAGS=--document-hidden-items ./x doc library/core.

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[bjorn3]

The UNICODE_VERSION constant is marked as stable. Is is actually usable on stable or should the #[stable] be removed?

[joboet]

The UNICODE_VERSION constant is marked as stable. Is is actually usable on stable or should the #[stable] be removed?

It's not usable as core::unicode::UNICODE_VERSION because the module is unstable, and both the associated and the freestanding stable UNICODE_VERSION constants are redefinitions, not reexports, so I think that yes, the attribute should be removed.

[clarfonthey]

According to rustdoc, it's actually marked as unstable, so, the stable attribute here is probably an artefact of the past. I can also confirm this on the current stable.

It's not necessary, but I'll add it to the PR.