If you build your interface with C#, you can use the XML comment that are actually built within the DLL. (/// thing) You'll be able to add remark and description of every parameters.
If you are using VB.NET... it can only be done with .NET 2.0 (find it stupid).
And last... no... you can't have interfaces with code within them. Interface is like a blueprint that people must follow. There is virual class that allow you to have partial code in there (much more like a incomplete class).
So that's it.
If you have any other questions, feel free to ask!