Rollup merge of rust-lang#90723 - asquared31415:box_docs, r=jyn514

Better document `Box` and `alloc::alloc::box_free` connection

The internal `alloc::alloc::box_free` function requires that its signature matches the `owned_box` struct's declaration, but previously that connection was only documented on the `box_free` function.

This PR makes the documentation two-way to help anyone making theoretical changes to `Box` to see the connection, since changes are more likely to originate from `Box`.

Author: matthiaskrgr

library/alloc/src/boxed.rs

@@ -169,6 +169,9 @@ use crate::vec::Vec;
 #[lang = "owned_box"]
 #[fundamental]
 #[stable(feature = "rust1", since = "1.0.0")]
+// The declaration of the `Box` struct must be kept in sync with the
+// `alloc::alloc::box_free` function or ICEs will happen. See the comment
+// on `box_free` for more details.
 pub struct Box<
     T: ?Sized,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,