Rollup merge of #40335 - tbu-:pr_doc_str_to_somecase, r=steveklabnik
Rollup merge of #40335 - tbu-:pr_doc_str_to_somecase, r=steveklabnik

Document why `str.to_{lower,upper}case` return `String`

Fixes #39201.

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