Update CS0524 documentation to reflect C# 8.0 changes allowing nested types in interfaces

[Copilot]

The CS0524 compiler error documentation was outdated and misleading. It stated that interfaces cannot declare types, but this restriction was lifted in C# 8.0 with the introduction of default interface members.

Changes Made

• Updated the documentation to clarify that CS0524 is no longer generated for nested types in interfaces since C# 8.0
• Explained that nested types are now allowed as part of the default interface members feature
• Changed the example from showing an error case to showing valid modern C# code
• Fixed the interface naming convention in the example (changed Clx to IExample following C# conventions)
• Updated the ms.date field to reflect the documentation update
• Added helpful cross-references to related documentation about interfaces, nested types, and C# version history

Before and After

Before: The documentation incorrectly stated that interfaces cannot contain user-defined types and showed an example that would generate CS0524.

After: The documentation accurately explains that this restriction was removed in C# 8.0 and provides a working example of nested types in interfaces.

The code example now compiles and runs successfully in modern C#:

// This code is valid in C# 8.0 and later
public interface IExample  
{  
    public class NestedClass   // Valid since C# 8.0
    {  
    }  
}  

Fixes #28427.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/misc/cs0524.md	docs/csharp/misc/cs0524

[BillWagner]

One change, and then this is ready.

[BillWagner]

This is now ready for final review.