Add ability to have both public and private definitions
I'm currently documenting libfprint: https://fprint.freedesktop.org/libfprint-dev/
It has an external public API, which front-ends (mainly the
fprintd D-Bus daemon and test applications) use, and an internal API to be used by drivers. A number of structures are opaque in the public API, but members are accessible in the private API.
While keeping all the symbols in the same manual, to make browsing easier, I'd like a way to document both the internal and external structures.