Readonly Structs are not shown as 'readonly struct'

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


I expected:

```cs
public readonly struct DateTime : IComparable, IComparable, IConvertible, IEquatable, IFormattable, System.Runtime.Serialization.ISerializable
```

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