Add note about NRT runtime behavior changes in libraries

[Copilot]

The nullable reference types documentation previously gave the impression that enabling NRT would never change runtime behavior. However, libraries like Entity Framework Core use reflection to read nullable attributes and modify their behavior accordingly.

The issue:

• Without NRT: EF Core maps reference type properties as optional in the database unless RequiredAttribute is explicitly added
• With NRT: EF Core maps reference type properties as required in the database unless marked with nullable annotation (?)

The fix:
Added an Important callout in the "Nullable references and static analysis" section of the language reference article to clarify that while the C# compiler doesn't introduce behavior changes, other libraries may use reflection to produce different runtime behavior based on nullable annotations.

The note specifically mentions Entity Framework Core as an example, explaining that it interprets nullable references as optional values and non-nullable references as required values.

This addresses the documentation gap and helps developers understand when NRT might indirectly affect runtime behavior through library implementations.

Fixes #19633.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/builtin-types/nullable-reference-types.md	docs/csharp/language-reference/builtin-types/nullable-reference-types

[BillWagner]

I had one wording suggestion, and then this is ready to merge

[BillWagner]

This is ready for final review.

@IEvangelist PTAL