Document slice patterns

[Centril]

Tracking issue: rust-lang/rust#62254
Stabilization PR & report: rust-lang/rust#67712 (comment)
Version: 1.42.0.

The most relevant content here is the actual report.

[mark-i-m]

@Centril Would you be up to write such a chapter? Otherwise, realistically, this will likely remain an issue on this repo, as we have a lot of backlog. (An issue is still better than nothing, though, I guess).