Rollup merge of #39299 - federicomenaquintero:master, r=GuillaumeGomez
Rollup merge of #39299 - federicomenaquintero:master, r=GuillaumeGomez

In std:rc, clarify the lack of mutability inside an Rc

Also, point to the example in Cell's docs for how to do it.

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