Re-add some justification

Author: 5225225

library/core/src/mem/mod.rs

@@ -666,6 +666,11 @@ pub unsafe fn zeroed<T>() -> T {
 /// As the [`assume_init` documentation][assume_init] explains,
 /// [the Rust compiler assumes][inv] that values are properly initialized.
 ///
+/// Truly uninitialized memory like what gets returned here
+/// is special in that the compiler knows that it does not have a fixed value.
+/// This makes it undefined behavior to have uninitialized data in a variable even
+/// if that variable has an integer type.
+///
 /// Therefore, it is immediate undefined behavior to call this function on nearly all types,
 /// including integer types and arrays of integer types, and even if the result is unused.
 ///