Improve in parameter modifier example with meaningful struct-based demonstration

[Copilot]

Fixes #25422

Problem

The current example for the in parameter modifier uses a simple int parameter, which doesn't effectively demonstrate the purpose and benefits of the in modifier. As pointed out in the issue:

• Without the in keyword, the value would still be 44 (since int is a value type)
• The example doesn't show why you'd use in in the first place
• It only demonstrates that you can't modify the parameter (via commented code)

Solution

Replaced the inadequate int example with a comprehensive struct-based demonstration that addresses all concerns:

Before (Problematic):

int readonlyArgument = 44;
InArgExample(readonlyArgument);
Console.WriteLine(readonlyArgument);     // value is still 44

void InArgExample(in int number)
{
    // Uncomment the following line to see error CS8331
    //number = 19;
}

After (Improved):

var largeStruct = new LargeStruct { Value1 = 42, Value2 = 3.14, Value3 = "Hello" };

// Using 'in' avoids copying the large struct and prevents modification
ProcessLargeStruct(in largeStruct);
Console.WriteLine($"Original value unchanged: {largeStruct.Value1}");

// Without 'in', the struct would be copied (less efficient for large structs)
ProcessLargeStructByValue(largeStruct);
Console.WriteLine($"Original value still unchanged: {largeStruct.Value1}");

void ProcessLargeStruct(in LargeStruct data)
{
    // Can read the values
    Console.WriteLine($"Processing: {data.Value1}, {data.Value2}, {data.Value3}");
    
    // Uncomment the following line to see error CS8331
    // data.Value1 = 99; // Compilation error: cannot assign to 'in' parameter
}

void ProcessLargeStructByValue(LargeStruct data)
{
    // This method receives a copy of the struct
    Console.WriteLine($"Processing copy: {data.Value1}, {data.Value2}, {data.Value3}");
    
    // Modifying the copy doesn't affect the original
    data.Value1 = 99;
}

What the New Example Demonstrates

1. Why use in: Performance benefits when passing large structs (avoids copying)
2. Realistic scenario: Processing data without needing to modify it
3. Clear comparison: Shows both in parameter and regular parameter methods side-by-side
4. Immutability: Demonstrates that in parameters cannot be modified
5. Educational value: Enhanced comments explain the behavior and benefits

Changes Made

• Updated FirstInExample() method in RefParameterModifier.cs with meaningful struct-based example
• Added LargeStruct definition with multiple fields to demonstrate performance benefits
• Enhanced comments explaining the purpose and benefits of in modifier
• Maintained backward compatibility with existing documentation structure

The example now clearly shows why developers would choose to use the in modifier and provides a practical, educational demonstration of its benefits.

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[BillWagner]

@copilot Please start working on this again.

[BillWagner]

@copilot Can you start working on this again?

[BillWagner]

@copilot Can you start working on this again?

[BillWagner]

This is ready. I'll mark it ready for final review.

[BillWagner]

The snippets failure is related to a preview feature.