When you release something as open source, you don't post it to multiple code hosting sites. You typically would post it to just one or just have a website of your own where you make the code available for download.
Codeplex is a pretty common and popular option for open-source .NET applications. In addition to hosting the code, they also have support for discussion areas, a description of your application, and some level of defect tracking.
You will need to decide on the license for your code, which will define how other people are allowed to use it. I think Codeplex offers several options to choose from, but I'm not entirely sure.
As far as documentation goes, it's certainly a good idea to create some if you want to encourage people to change your code. It's not strictly necessary, but it sure wouldn't hurt. You'll also want to document other things like installation and configuration options for people who don't want to tinker with your code and just want to use the forum as is.