lsifstore: add ability to query API docs search index - take 2

Created by: slimsag

I had previously sent these exact commits in https://github.com/sourcegraph/sourcegraph/pull/26487 but as I learned more the PR grew, and I wanted to land more backend tests first, so I closed it and sent:

• https://github.com/sourcegraph/sourcegraph/pull/26736
• https://github.com/sourcegraph/sourcegraph/pull/26913
• https://github.com/sourcegraph/sourcegraph/pull/26954

Now that those are merged, it's time to land this.