generated doc do not please nitpicky.

[ninoles]

Adding the -n options to sphinx-build will generate a warning to all properties. Just taking the GithubUser in the README.rst example, this is what I got:

E:\Test\jsondomain\doc\source\jsonobj.rst:: WARNING: json:prop reference target not found: login
E:\Test\jsondomain\doc\source\jsonobj.rst:: WARNING: json:prop reference target not found: id
E:\Test\jsondomain\doc\source\jsonobj.rst:: WARNING: json:prop reference target not found: avatar_url
E:\Test\jsondomain\doc\source\jsonobj.rst:: WARNING: json:prop reference target not found: gravatar_url

My impression is that the make_xref should be override to create proper xref for props, (like "GithubUser.login" instead of just "login") but I'm not sure since I cannot see where the reference are created (the bug happen even with :noindex: set).