updated documentation?

The documentation team is aware of this, but I'm not sure when the changes will be made.

It sounds lame, but currently the best way to find out which fields are exposed is the source code:
That's actually not as bad as it sounds. The Lua interfaces are in files name <something>-lua.c where <something>
is 'network-backends' for example.
To pick the proxy.global.backends structure as an example:

The annotated source file for network-backend-lua.c is at http://bazaar.launchpad.net/%7Emysql-proxy-developers/mysql-proxy/trunk/annotate/head%3A/src/network-backend-lua.c
Very close to the top of the file, there's the function proxy_backend_get, that handles the access to the Lua 'table' proxy.global.backends[i].
You can easily see which structures are accessible from there.

The common naming scheme for various structures in MySQL Proxy is network-'something'.c for the actual low-level implementation and network-'something'-lua.c for the Lua interface code.

I realize it's inconvenient to look at code rather than documentation, but given the pace we still change interfaces we cannot expect the documentation team to pick up as quickly (given their other responsibilities).
This will naturally change with a GA release.