Rollup merge of #40619 - stjepang:unstable-book-sort-unstable, r=frewsxcv
Rollup merge of #40619 - stjepang:unstable-book-sort-unstable, r=frewsxcv

Add docs for sort_unstable to unstable book

Tracking issue for the feature: #40585

r? @steveklabnik

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust