Rollup merge of #130214 - RalfJung:zeroed, r=Mark-Simulacrum

MaybeUninit::zeroed: mention that padding is not zeroed

That should clarify cases like [this](#129778 (comment)).

library/core/src/mem/maybe_uninit.rs

@@ -351,6 +351,9 @@ impl<T> MaybeUninit<T> {
     /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
     /// be null.
     ///
+    /// Note that if `T` has padding bytes, those bytes are *not* preserved when the
+    /// `MaybeUninit<T>` value is returned from this function, so those bytes will *not* be zeroed.
+    ///
     /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
     /// It is your responsibility to make sure `T` gets dropped if it got initialized.
     ///