hplip ships with auxiliary documentation in /usr/local/share/doc/hplip. It also installs numerous programs as hp-xxx in /usr/local/bin. None of the installed hp-xxx programs has a man page. They should at least have a minimal man page referencing the installed documentation and telling you to run "hp-xxx -h". Otherwise the user has to grep around or look at the port pkg-plist to see what and where the documentation is, particularly when you don't know which package installed it. Better would be to have an actual man page. Perhaps the output of "hp-xxx -h" could be used to generate a minimal man page.
This is not a FreeBSD bug, you'll have to report this upstream. Some documentation is available at https://developers.hp.com/hp-linux-imaging-and-printing/tech_docs/man_pages/setup
I am closing this issue, as it is not a bug with FreeBSD, but with the upstream project who don't provide manpages. Please do follow up with a bug report to the hplip upstream project https://launchpad.net/hplip/+filebug.
I've already filed a bug with upstream, sounds like they will get it done.