Readonly Members are not shown as 'readonly'

If a member in a struct is readonly it should show this in the docs. Take a look (for example) at [System.Drawing.Point.ToString()](https://docs.microsoft.com/en-us/dotnet/api/system.drawing.point.tostring?view=net-5.0#System_Drawing_Point_ToString) (.NET 5.0)


I expected

```cs
public readonly override string ToString ();
```

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