alloc: restrict impl ZeroableOption for Box to T: Sized

Similar to what was done for `Zeroable<NonNull<T>>` in commit 9caa350
("last docs changes and zeroable unsized pointer fixes"), the latest Rust
documentation [1] says it guarantees that `transmute::<_, Option<T>>([0u8;
size_of::<T>()])` is sound and produces `Option::<T>::None` only in some
cases. In particular, it says:

    `Box<U>` (specifically, only `Box<U, Global>`) when `U: Sized`

Thus restrict the `impl` to `Sized`, and use similar wording as in that
commit too.

Link: https://doc.rust-lang.org/stable/std/option/index.html#representation [1]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

Diff for: src/alloc.rs

@@ -17,11 +17,9 @@ use crate::{
 
 pub extern crate alloc;
 
-// SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
-//
-// In this case we are allowed to use `T: ?Sized`, since all zeros is the `None` variant and there
-// is no problem with a VTABLE pointer being null.
-unsafe impl<T: ?Sized> ZeroableOption for Box<T> {}
+// SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee:
+// https://doc.rust-lang.org/stable/std/option/index.html#representation).
+unsafe impl<T> ZeroableOption for Box<T> {}
 
 /// Smart pointer that can initialize memory in-place.
 pub trait InPlaceInit<T>: Sized {