Clarify Optional Parameters and Nullable Optional Parameter Behavior in C#.

[shethaadit]

Summary

This PR improves the documentation for optional parameters in C# by:

• Clarifying the distinction between nullable (T?) parameters and optional parameters with default values.
• Highlighting that parameters can be both nullable and optional (T? allTheOptionalMeanings = default).
• Adding examples and notes to address user confusion, ensuring better understanding of the behavior of nullable and optional parameters.
• Maintaining all existing code snippets as they are already accurate and illustrative.

Fixes #44404

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/classes-and-structs/named-and-optional-arguments.md	Named and Optional Arguments (C# Programming Guide)

[BillWagner]

Hi @shethaadit

The changes you've made emphasize the confusion rather than fixing it. A method signature can set the "default" value for an optional parameter. That's a different concept than the "default value" for a type. It also can be different than specifying a "missing" value. Consider this example:

public class Buffer<T>
{
    private T[] _storage;
    
    public Buffer(int size = 10)
    {
        _storage = new T[size];
    }
}

The "default value" for size is 10. That's different than the default value for the int type (that's 0).

[BillWagner]

The new sentence emphasizes the cause of the confusion, rather than removing it. We need to make a clear distinction between an optional parameter and the default value for a type.

[BillWagner]

The additional sentence here mixes an optional parameter with the default value for a type.

[BillWagner]

This isn't correct. All of these declarations are legal. They conform to the first three bullet points:

public struct S
{
}

public class C {
    void M(int? size = 5){} // a constant
    void M(string? msg = "Hello World"){} // constant
    void M(S someStruct = new S()){} // new expression, assuming `S` is a `struct`
    void M(object? o = default) { } // OK default expression
    void M(int size, S someStruct = default) {} // OK default expression addition parameter needed
    void M(S? someStruct = default) {} // OK default expression
}

[shethaadit]

Hi @BillWagner, could you please check once if the updates makes sense?