Slight improvement of support for C# code

After fixing the c# crashing I found that Fsharp.Formatting really didn't add any xmldoc comments to the generated API docs (MetadataFormat).

After some poking around I found that FSC consistently returned and empty string when `xmlDocSig` was requested from types and members. 
I suppose such are to be expected as FSC is _F#_ specific, so I created a small workaround.

Unfortunately it is in no way complete, for instance it only seems to work with _Properties_ and _Types_.
FSC fails reporting any methods from analyzing C# assemblies. 

But, this is still an improvement I think (albeit a small and incomplete one..)