Expand description
Propagates stability to child items.
The purpose of this pass is to make items whose parents are “more unstable”
than the item itself inherit the parent’s stability.
For example, core::error::Error is marked as stable since 1.0.0, but the
core::error module is marked as stable since 1.81.0, so we want to show
core::error::Error as stable since 1.81.0 as well.