My job requires me to do exactly what you're looking to do but for multiple companies/networks. Then, as soon as I'm done, I usually pack up and go or get hired in and fix the network.
Since I'm writing the Network Overview for managers AND potential future network managers I tend to write mine in the following format:
1) Synopsis of what the network does for the company, what general technologies they use (Windows AD vs *nix OD, thin clients vs Windows boxes, Cisco vs Brand X), and what the LOB software is.
2) Points of contact for the ISP and other providers (anti-spam, anti-virus, hardware, etc). Passwords for various accounts and services.
3) Logical network overview map (visio), containing firewalls, routers, switches, other devices, open/forwarded ports, IPs, what the servers do, what vlans are in place, Quick explainations for why (such as why vlan vs a seperate subnet).
4) Physical map of devices if the complexity of the network calls for it.
5) Software notes, what apps are critical for the business and which systems they rely on.
Then, for my specific job I have to do the following:
6) Licensing issues.
7) Network weaknesses/points of failure.
9) Other rec's.