Rollup merge of #48201 - NovemberZulu:master, r=steveklabnik
Rollup merge of #48201 - NovemberZulu:master, r=steveklabnik

rephrase UnsafeCell doc

As shown by discussions on users.rust-lang.org [[1]], [[2]], UnsafeCell doc is not totally clear. I tried to made the doc univocal regarding what is allowed and what is not. The edits are based on my understanding following [[1]].

[1]: https://users.rust-lang.org/t/unsafecell-behavior-details/1560
[2]: https://users.rust-lang.org/t/is-there-a-better-way-to-overload-index-indexmut-for-a-rc-refcell/15591/12

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