As far as I can tell, installing Unity via the .deb package didn't install the offline documentation anywhere, which is a feature I'm very accustomed to having. Did I just miss it, or is it not installed? If it isn't installed, is there a way to download it from elsewhere, or do I have to copy it from a Windows installation? I'd prefer a way to keep an up-to-date version on disk without much maintenance.
The documentation is not installed with the editor. You'll have to copy it from a Windows (or Mac) installation. If you download the installer for Windows, it asks you which parts you want to install. You could uncheck everything other than the offline docs, and install it in Wine (and copy/link it to the appropriate location).