Rollup merge of rust-lang#134606 - RalfJung:ptr-copy-docs, r=Mark-Simulacrum

ptr::copy: fix docs for the overlapping case

Fixes rust-lang/unsafe-code-guidelines#549

As discussed in that issue, it doesn't make any sense for `copy` to read a byte via `src` after it was already written via `dst`. The entire point of this method is that is copies correctly even if they overlap, and that requires always reading any given location before writing it.

Cc `@rust-lang/opsem`

Author: jieyouxu

core/src/intrinsics/mod.rs

@@ -4364,13 +4364,11 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * `src` must be [valid] for reads of `count * size_of::<T>()` bytes, and must remain valid even
-///   when `dst` is written for `count * size_of::<T>()` bytes. (This means if the memory ranges
-///   overlap, the two pointers must not be subject to aliasing restrictions relative to each
-///   other.)
+/// * `src` must be [valid] for reads of `count * size_of::<T>()` bytes.
 ///
 /// * `dst` must be [valid] for writes of `count * size_of::<T>()` bytes, and must remain valid even
-///   when `src` is read for `count * size_of::<T>()` bytes.
+///   when `src` is read for `count * size_of::<T>()` bytes. (This means if the memory ranges
+///   overlap, the `dst` pointer must not be invalidated by `src` reads.)
 ///
 /// * Both `src` and `dst` must be properly aligned.
 ///