docs: Document CS8080 private accessor override limitation

[Copilot]

CS8080 was nearly undocumented—the VS error link led to a "no specifics" page, and the existing docs entry didn't cover the common case that triggers it: a base property with a private accessor.

Changes

• property-declaration-errors.md — CS8080 bullet: Expanded the existing entry to explain that when the base class property has a private accessor (e.g., private set), an auto-implemented override is impossible because private members aren't accessible in derived classes. Adds explicit guidance to use explicit accessor bodies instead:

  class Base {
      public virtual int Member { get; private set; }
  }
  
  // ❌ CS8080 — auto-implemented can't satisfy the private set
  class Derived : Base {
      public override int Member { get; }
  }
  
  // ✅ Use explicit accessor body instead
  class Derived : Base {
      public override int Member { get => base.Member; }
  }

• Fixes Cannot override property with private set #27712

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/property-declaration-errors.md	docs/csharp/language-reference/compiler-messages/property-declaration-errors

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