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

---

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).

[shethaadit]

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

[BillWagner]

Thanks again @shethaadit

This LGTM, and I'll now.