Rollup merge of #40379 - clarcharr:box_docs, r=brson

Box docs: no allocation is done for ZSTs.

Updated to add a small bit saying that ZSTs don't actually allocate on `Box::new`.

src/liballoc/boxed.rs

@@ -225,6 +225,8 @@ impl<T: ?Sized> Drop for IntermediateBox<T> {
 impl<T> Box<T> {
     /// Allocates memory on the heap and then places `x` into it.
     ///
+    /// This doesn't actually allocate if `T` is zero-sized.
+    ///
     /// # Examples
     ///
     /// ```