rustdoc: item-info is incorrectly wrapped on narrow screens

[jsha]

Steps to reproduce:

1. Visit https://doc.rust-lang.org/nightly/std/os/unix/ucred/struct.UCred.html#structfield.pid
2. Make the browser window narrow until the item-info ("This is a nightly-only experimental API...") starts wrapping.

Expected result: item-info wraps as normal text.

Actual result: item-info wraps like a table. See screenshot: