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

remove confusing remarks about mixed volatile and non-volatile accesses

These comments were originally added by @ecstatic-morse in rust-lang@911d35f 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](rust-lang#58599 (comment)), which -- to my knowledge -- they are not. So better remove this.

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

src/libcore/ptr.rs

@@ -810,9 +810,6 @@ pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
 /// to not be elided or reordered by the compiler across other volatile
 /// operations.
 ///
-/// Memory accessed with `read_volatile` or [`write_volatile`] should not be
-/// accessed with non-volatile operations.
-///
 /// [`write_volatile`]: ./fn.write_volatile.html
 ///
 /// # Notes
@@ -881,9 +878,6 @@ pub unsafe fn read_volatile<T>(src: *const T) -> T {
 /// to not be elided or reordered by the compiler across other volatile
 /// operations.
 ///
-/// Memory accessed with [`read_volatile`] or `write_volatile` should not be
-/// accessed with non-volatile operations.
-///
 /// `write_volatile` does not drop the contents of `dst`. This is safe, but it
 /// could leak allocations or resources, so care should be taken not to overwrite
 /// an object that should be dropped.