Rollup merge of #44497 - tommyip:doc_example, r=frewsxcv
Rollup merge of #44497 - tommyip:doc_example, r=frewsxcv

Add doc example to str::from_boxed_utf8_unchecked

Fixes #44463.

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