Rollup merge of #36311 - frewsxcv:instant-elapsed-example, r=GuillaumeGomez
Rollup merge of #36311 - frewsxcv:instant-elapsed-example, r=GuillaumeGomez

Add doc example for `std::time::Instant::elapsed`.

None

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