Hi Jamie!
The Quick Navigation feature uses a fuzzy search algorithm to find matches. At the moment, it only supports searching across the titles of the different pages, that's why you don't get results for "Default Parameter Values", since this is a subtitle inside one of the pages' content.
For example, if you search for init in the TSPL docs, you’ll get initialization, deinitialization, inheritance… which all correspond to other page titles.
When implementing this feature, we used the index.json file as the data input, which only contains the data needed to render the sidebar, so no other information is available for performing the search. Back then, when we tried to add more data as input for the search we faced some performance issues.
I'm not aware of any open issue regarding this feature request, so feel free to open an issue at github.com/swiftlang/swift-docc-render. This was implemented a few years ago in the open source, so you can track the entire development of it in this forum post: forums.swift.org/t/pitch-quick-navigation-in-docc-render/59550.