contrib.admindocs will only document template tags and functions that can be invoked from templates. Therefore it has limited use. It would be really useful to be able to document all of the functions in the models (not just the ones that don't take any arguments). contrib.admindocs is a neat way of creating some basic documentation without any extra effort other than docstrings. By adding this feature, this potential could be fulfilled. Basically, you would get at least some documentation, just by writing docstrings (which you should be doing in the first place).
Because there might be need to keep the current functionality, I suggest the following solution. By default the docs would stay the same as they are now (show only the functions that can be invoked from templates), but we would add a "Show more" button, which would show all the functions instead.
This makes it easy, because there is no need for an extra setting in settings.py and it allows both new extended functionality and old functionality.
PS This is my first time suggesting a new feature, so please let me know, if I missed something!