Document generated type-contract diagnostics for records and closed types

Two diagnostics are undocumented but fit cleanly into existing article families: `CS9391` (a record member must be declared explicitly because a base is abstract) belongs with record declaration errors, and `CS9395` (the `IsClosedTypeAttribute.DerivedTypes` property must be an instance property with public get/set, no parameters, and type `System.Type[]`) belongs with the closed/union diagnostic family.

Add `CS9391` to `record-declaration-errors.md`, keeping it tightly focused on record synthesis and inheritance-contract behavior. Add `CS9395` to `union-declaration-errors.md`, explaining the required `DerivedTypes` metadata shape in the same concise style as adjacent closed/union diagnostics. Both codes should come out of "undocumented" status with specific, article-appropriate coverage. See [Roslyn error codes](https://github.com/dotnet/roslyn/blob/main/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs) for exact message text.

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[Associated WorkItem - 632362](https://dev.azure.com/msft-skilling/Content/_workitems/edit/632362)