As I suspected the documentation for use and deployment of FPDoc is pretty thin in the Wiki, but I'll try to investigate and improve the Wiki if I have time.
In the wiki yes it's pretty thin, but you should have a full
fpdoc.chm in the chm docs folder of your Lazarus installation. And a quite complete one, at that
You can also consult it online, at:
https://www.freepascal.org/docs-html/current/fpdoc/fpdoc.html