rustdoc: item-infos have different vertical size

[jsha]

On pages that have an item-info for "experimental" and also a platform restriction (e.g. https://doc.rust-lang.org/nightly/std/arch/macro.is_arm_feature_detected.html), the two item-infos are different sizes:

The platform restriction should be the same height as the experimental item-info.