How to install the Vala Reference Manual into Devhelp using the apt package manager
The Vala Documentation states that the Vala Reference Manual is available in HTML and PDF versions and “is also available for your installed version of Vala as DevHelp from your distribution, such as Fedora or Ubuntu.”
Sadly, though, it doesn’t tell you the package names or link to the packages.
For Debian’s apt-based systems like Pop!_OS and Ubuntu, the package you’re looking for is called vala-0.40-doc.
To install the Vala Reference Manual into DevHelp, run the following command:
sudo apt install vala-0.40-doc
In case you’re reading this far into the future and there is a different version of Vala and a different version of the package, you can search for the latest version with the following command:
apt search 'vala-[0-9]+\.[0-9]+-doc'