Bug #22704 is fixed, so while I get the notification box that tells me that the manual is not installed I can simply click on the link to open it in a browser.
It also tells me to go to Preferences if I want to always open the manual in my browser, which works fine. The only way to include the manual is to download the wiki, and we can’t do that reproducibly, so we cannot make it a fixed output derivation. As it’s not feasible for us to include the manual, I’m closing this issue. -- Ricardo