Update CS0524 documentation to reflect C# 8.0 changes allowing nested types in interfaces (#47669)

* Initial plan

* Update CS0524 documentation to reflect C# 8.0 changes

Co-authored-by: BillWagner <[email protected]>

* Update ms.date to 07/31/2025 as requested

Co-authored-by: BillWagner <[email protected]>

* Improve wording as suggested by @gewarren

Co-authored-by: gewarren <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: gewarren <[email protected]>

Author: 3 people

docs/csharp/misc/cs0524.md

@@ -1,7 +1,7 @@
 ---
 description: "Compiler Error CS0524"
 title: "Compiler Error CS0524"
-ms.date: 07/20/2015
+ms.date: 07/31/2025
 f1_keywords: 
   - "CS0524"
 helpviewer_keywords: 
@@ -12,18 +12,26 @@ ms.assetid: a5cd8fb0-f5df-4580-9116-a6be4dffd1cb
 
 'type' : interfaces cannot declare types  
 
- An [interface](../language-reference/keywords/interface.md) cannot contain a user-defined type; it should contain only methods and properties.  
-  
-## Example  
+Prior to C# 8.0, an [interface](../language-reference/keywords/interface.md) could not contain user-defined types. Starting with C# 8.0, interfaces can declare [nested types](../programming-guide/classes-and-structs/nested-types.md) as part of the default interface members feature.
+
+In modern C# (C# 8.0 and later), this error is no longer generated for nested types in interfaces. The compiler now allows interfaces to contain nested classes, structs, interfaces, enums, and delegates.
+
+## Historical example
 
- The following sample generates CS0524:  
+The following sample generates CS0524 in C# versions prior to 8.0, but is valid in modern C# versions:  
 
 ```csharp  
-// CS0524.cs  
-public interface Clx  
+// This code is valid in C# 8.0 and later
+public interface IExample  
 {  
-    public class Cly   // CS0524, delete user-defined type  
+    public class NestedClass   // Valid since C# 8.0
     {  
     }  
 }  
 ```
+
+## See also
+
+- [Interfaces](../language-reference/keywords/interface.md)  
+- [Nested types](../programming-guide/classes-and-structs/nested-types.md)  
+- [What's new in C# version history](../whats-new/csharp-version-history.md#c-version-80)