Improve CS0430 extern alias documentation with practical usage examples (#47708)

* Initial plan

* Improve CS0430 extern alias example to show actual usage

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

* Add cross-reference to extern alias documentation

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

---------

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

Author: Copilot

docs/csharp/misc/cs0430.md

@@ -19,7 +19,10 @@ The extern alias 'alias' was not specified in a /reference option
 ```csharp  
 // CS0430_a.cs  
 // compile with: /target:library
-public class MyClass {}  
+public class MyClass
+{
+    public static void M() { /*...*/ }
+}  
 ```  
 
 ## Example 2  
@@ -31,6 +34,10 @@ public class MyClass {}
 extern alias MyType;   // CS0430  
 public class Test
 {  
-   public static void Main() {}  
+   public static void Main() { MyType::MyClass.M(); }  
 }  
 ```
+
+## See also
+
+- [extern alias](../language-reference/keywords/extern-alias.md)