Rollup merge of #60972 - RalfJung:volatile, r=alexcrichton
Rollup merge of #60972 - RalfJung:volatile, r=alexcrichton

remove confusing remarks about mixed volatile and non-volatile accesses

These comments were originally added by @ecstatic-morse in https://github.com/rust-lang/rust/commit/911d35f0bfd207112806eaec2763201dad06d1c7 and then later edited by me. The intention, I think, was to make sure people do both their reads and writes with these methods if the affected memory really is used for communication with external devices.

However, [people read this as saying that mixed volatile/non-volatile accesses are UB](https://github.com/rust-lang/rust/issues/58599#issuecomment-493791130), which -- to my knowledge -- they are not. So better remove this.

Cc @rkruppe @rust-lang/wg-unsafe-code-guidelines

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