Rollup merge of #31515 - steveklabnik:doc_drain, r=alexcrichton
Rollup merge of #31515 - steveklabnik:doc_drain, r=alexcrichton

This is the last bit of String docs needed to

Close #29376

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