Rollup merge of #52668 - RalfJung:ptr-doc, r=cramertj
Rollup merge of #52668 - RalfJung:ptr-doc, r=cramertj

clarify pointer offset function safety concerns

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