Some support for message type documentation and message-only packages went into rosdoc2 not that long ago via Support msgs, services, and actions by rkent · Pull Request #76 · ros-infrastructure/rosdoc2 · GitHub and Support only messages by rkent · Pull Request #78 · ros-infrastructure/rosdoc2 · GitHub (although i have not tried using that myself yet)
That functionality might not be documented yet.
Also add me to the list of people initially confused about the rosdoc2 installation. It’s easy enough to do and also nice that it’s python-only without specific ros dependencies, but then again i would have expected it to be a ros package, buildable with colcon and installable via apt install ros-DISTRO-rosdoc2
(which reminds me that colcon doc would be nice to have, i think i saw some ideas about that before 
)