Rollup merge of #85715 - fee1-dead:document-string, r=JohnTitor
Rollup merge of #85715 - fee1-dead:document-string, r=JohnTitor

Document `From` impls in string.rs

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