Rollup merge of #38208 - frewsxcv:btreesetdocs, r=alexcrichton
Rollup merge of #38208 - frewsxcv:btreesetdocs, r=alexcrichton

Improvements to `BTreeSet` documentation.

None

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