Moved details of unstable non-ascii identifiers from the Reference

[brauliobz]

Moved details of unstable non-ascii identifiers from the Reference to the Unstable book

r? @steveklabnik

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

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[steveklabnik]

This looks good to me, but

[01:27:20] unstable-book/print.html:1496: broken link - unstable-book/keywords.html

[01:27:20] unstable-book/language-features/non-ascii-idents.html:123: broken link - unstable-book/keywords.html

you'll need to change the link to something like ../../reference/keywords.html#strict-keywords

[kennytm]

CI passed, assuming good to go.

@bors r=steveklabnik rollup

[bors]

📌 Commit 8e6119f has been approved by steveklabnik