Define "defining declaration" and enhance CS0759 compiler error documentation

[Copilot]

The CS0759 compiler error documentation was unclear about what a "defining declaration" means, leaving users confused when encountering this error. The existing documentation simply stated "No defining declaration found..." without explaining the concept or providing working examples.

Changes Made

Enhanced Documentation: Added comprehensive explanation for CS0759 that:

• References the existing definition of "defining declaration" from the Partial members section
• Explains the two required parts: defining declaration (signature without body) and implementing declaration (signature with body)
• Clearly states when CS0759 occurs: having an implementation without a corresponding definition

Code Examples: Created practical examples showing:

Incorrect code that generates CS0759:

public partial class ExampleClass
{
    // ERROR: This is an implementing declaration without a corresponding defining declaration
    partial void MyMethod() // CS0759
    {
        Console.WriteLine("Implementation without definition");
    }
}

Correct fix - separate partial declarations:

public partial class CorrectExampleClass
{
    // Defining declaration (signature without body)
    partial void MyMethod();
}

public partial class CorrectExampleClass  
{
    // Implementing declaration (signature with body)
    partial void MyMethod()
    {
        Console.WriteLine("This works correctly");
    }
}

Alternative fix - both in same partial class:

public partial class AlternativeExampleClass
{
    partial void MyMethod();        // Defining declaration
    
    partial void MyMethod()         // Implementing declaration
    {
        Console.WriteLine("This also works correctly");
    }
}

The enhanced documentation now clearly explains what a "defining declaration" is, why the CS0759 error occurs, and provides practical examples of both the error scenario and its resolution.

Fixes #27997.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/partial-declarations.md	docs/csharp/language-reference/compiler-messages/partial-declarations

[BillWagner]

One small change and this is ready.

[BillWagner]

This LGTM. It's ready for final review.