Add generic type argument accessibility example to CS0050 documentation

[Copilot]

The CS0050 compiler error documentation was missing an important case where the error can occur. The issue reported that CS0050 can also be caused when a generic type's type argument has lower accessibility than the method itself.

For example, this code generates CS0050:

internal class CeisData // Internal class
{
    public string Name { get; set; }
}

public class MyClass
{
    public static ObservableCollection<CeisData> BuildCeis()   // CS0050
    {
        return new ObservableCollection<CeisData>();
    }
}

The error occurs because CeisData is internal but the method BuildCeis() is public, making the type argument less accessible than the method.

This PR adds:

• Updated the main description to clarify that type arguments of generic types are also covered by the accessibility rule
• A new "Another cause" section with a complete example demonstrating the generic type argument accessibility issue
• A corrected version showing how to fix the issue by making the type argument appropriately accessible

The documentation now comprehensively covers both the original case (direct return type accessibility) and this additional case involving generic type arguments.

Fixes #31062.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/cs0050.md	Compiler Error CS0050

[BillWagner]

This is ready for final review.

[learn-build-service-prod]

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