While you could very well go about manually documenting every piece of the network, and hope it remains relevant and up to date in the future, this could take weeks and add significant overhead to your role in keeping current. I recommend looking into the many auto-discovery tools available from vendors like HP, BMC, Computer Associates, etc... They aim to store everything in a single database (CMDB) and track any changes or additions by scheduling delta-discoveries whenever you deem fit. The initial setup can be a lot of work, but since you know most of the information required by the auto-discovery tools for accessing system information (usernames, passwords, IP subnets, common services, ports...), it should be pretty straight forward for you. In a larger organization where this information is spread around various groups, it can be a lot more challenging.
HP has a product, formerly by a company called Mercury, that I find works quite well. It would at least be a good place to start looking... Link
here.
Good luck!