Clarify MaybeUninit::zeroed padding docs Closes rust-lang/rust#111608. This clarifies that padding bytes in `MaybeUninit::zeroed()` are not guaranteed to remain zeroed when the `MaybeUninit<T>` value is returned, matching the existing `mem::zeroed` documentation that padding is not necessarily zeroed. Verification: - `git diff --check` - `./x fmt --check` - `./x test tidy` - `./x test library/core --doc`