Reword {ptr,mem}::replace docs.

Fixes #50657.

Author: frewsxcv

src/libcore/mem.rs

@@ -635,8 +635,9 @@ pub fn swap<T>(x: &mut T, y: &mut T) {
     }
 }
 
-/// Replaces the value at a mutable location with a new one, returning the old value, without
-/// deinitializing either one.
+/// Moves `src` into the referenced `dest`, returning the previous `dest` value.
+///
+/// Neither value is dropped.
 ///
 /// # Examples
 ///

src/libcore/ptr.rs

@@ -239,8 +239,9 @@ unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, len: usize) {
     }
 }
 
-/// Replaces the value at `dest` with `src`, returning the old
-/// value, without dropping either.
+/// Moves `src` into the pointed `dest`, returning the previous `dest` value.
+///
+/// Neither value is dropped.
 ///
 /// # Safety
 ///