Schemadocs generates API documentation from the API facades schema json file. The output is a self contained html file that you can use in your browser to help navigate the types found with in juju.
To generate the html file, you need to ensure you have downloaded juju and have the files checked out correctly on your filesystem. Then run the following command (assuming you have go installed):
make output.html
Open the resulting output.html in your browser and enjoy.