I installed libguava-lib-doc from the Ubuntu store. But I can't figure out where it installed. Where are the docs, and how can I figure this out in general in the future?
The documentation you are looking for is in
Most (all?) packages install documentation into
/usr/share/doc/packages-name. If you're ever unsure what was installed by a package, you can run
dpkg -L package-name to get a list of files that were included.
You can find the files under: