Even though I mostly learned using Google, if you are using MS Visual C# or any C# developer that reads the DLL's Classes, the DLL is completely commented with it's functions. I recall being able to view every function in the DLL that I was using by doing something where the references were called (I think it was like right click->View functions or something like that).

Also, the Tao Framework simply converts the library of the normally not .NET compatible library to a library that is compatible. With that, the documentation should be downloaded from the source's site (such as Tao.OpenGL is really OpenGL so any documentation for OpenGL will work for it).

I know it's not a true solution, but it should help explain where to find documentation for the library you are using.