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

Rollup merge of #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`.