Rollup merge of #44453 - tommyip:doc_string_as_mut_str, r=frewsxcv
Rollup merge of #44453 - tommyip:doc_string_as_mut_str, r=frewsxcv

Add doc example to String::as_mut_str

Fixes #44429.

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