Ref Structs are not shown as 'ref struct'

If a struct is a 'ref struct' it should show this in the docs. Take a look (for example) at [Span](https://docs.microsoft.com/en-us/dotnet/api/system.span-1?view=netcore-3.1).


I expected:
```cs
public ref struct Span
```

The **ref** keyword is missing. This is a general problem for all ref structs in the framework.