Rollup merge of #37187 - frewsxcv:cow-doc-example, r=kmcallister
Rollup merge of #37187 - frewsxcv:cow-doc-example, r=kmcallister

Improve doc example for `std::borrow::Cow`.

None

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