adding multiline syntax for C#

I added the missing multi-line syntax for C#. It should now work with

```C#
/*
   A multiline comment
*/
```

as well as with

```C#
/**
 * Another multiline comment
 */
```

What's not supported is the (more often) used XML style documentation, like

```C#
/// <summary>
/// Log a given Exception
/// </summary>
/// <param name="ex">the exception object to log</param>
void Log(Exception ex);
```

But for support this kind of documentation I guess we'd have to exend groc further (definitely - if at all - subject of a different pull request ;) )