rustdoc: `Box` docs collapse implementations by default on nightly

[camelid]

This means that the docs are annoying to read and much less accessible for newcomers. Also, links to Box methods (such as Box::pin()) don't go to the method, and instead go to the top of the Box docs, with all the implementations collapsed.

nightly

beta (and stable)

[camelid]

Should be fixed on next nightly.