Update CS0173 documentation for C# 9 target-typed conditional expressions

[Copilot]

The CS0173 compiler error documentation was outdated and focused exclusively on class conversions, missing important information about C# 9's target-typed conditional expressions feature. This update addresses several issues:

Key Problems Fixed:

• Documentation claimed CS0173 only occurred with classes, but it can happen with any types
• Missing information about how C# 9's target-typed conditional expressions changed the behavior
• Outdated example with object o = (X == 0) ? null : null; that no longer produces CS0173
• No explanation of when CS8957 appears instead of CS0173 in different C# versions

Examples of the Issue:

// Still causes CS0173 in all versions (no target type with var)
var result = true ? 100 : "ABC";            

// Now works in C# 9+ due to target-typed conditional expressions  
object result = true ? 100 : "ABC";         // C# 8: CS8957, C# 9+: OK

Changes Made:

• Updated main description to cover all types, not just classes
• Added comprehensive explanation of C# 9 target-typed conditional expressions
• Replaced outdated examples with accurate, tested scenarios
• Added version-specific guidance for C# 8 vs C# 9+ behavior
• Included proper nullable reference type syntax (object?)
• Added note explaining when CS8957 appears instead of CS0173

The documentation now accurately reflects modern C# compiler behavior and provides clear guidance for resolving CS0173 across different language versions.

Fixes #40070.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/cs0173.md	Compiler Error CS0173

[BillWagner]

This is a great start @copilot, but I have some things for you to address.

[BillWagner]

This is ready for final review.