JRuby is missing manpages, at least for the main JRuby executable.

In the Debian package we ship a very old hand-generated manpage, but it's a struggle to keep it up to date.

The manpages could either be maintained in markdown or reStructured text format, then converted to groff at build-time, or, another common method of generating a manpage is using the help2man tool which builds a rudimentary manpage from the executable's --help information.

FWIW C-Ruby ships a great manpage: https://manpages.debian.org/trixie/ruby/ruby.1.en.html