Hidden all the way at the right end of the Google search app's tabs is 'In Apps' search.
As you might guess from the name, it allows you to search within the apps installed on your phone. Now Google has added Firebase App Indexing to the Drive Android app, allowing you to find your documents straight from the Google search app.
Once you have the latest Drive update installed, you should see your files appear along with results from other apps. Tapping the 'More' button will launch you into the Drive app for a more detailed view of the results.
While Google says that only the latest Drive update is required, there might be a server-side component that is still rolling out. If you don't have it right now, just wait a day or two