Rollup merge of #28048 - steveklabnik:doc_unimplemented, r=alexcrichton
Rollup merge of #28048 - steveklabnik:doc_unimplemented, r=alexcrichton


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