Fix overstatement about nullable reference types runtime protection

[Copilot]

The tutorial incorrectly stated that non-nullable reference type variables "can't be null" at runtime, which is misleading. While nullable reference types provide valuable compile-time warnings, they cannot prevent null values at runtime, especially for public APIs.

Changes Made

Primary fix (line 127):

• Before: "Its value can't be null."
• After: "While the compiler warns about potential null assignments, runtime null values are still possible. For public APIs, consider adding argument validation even for non-nullable reference types, since client code might not have nullable reference types enabled or could intentionally pass null."

Secondary improvement (line 206):

• Added clarification that "The compiler's static analysis helps ensure these design contracts are followed" to be more precise about the compiler's role.

Why This Matters

The original statement was problematic because:

1. Compiler warnings ≠ runtime protection
2. Public APIs can receive null values from callers who haven't opted into nullable reference types
3. Malicious or careless code can still pass null values
4. The overstatement could lead developers to skip necessary argument validation

The corrected documentation now accurately explains the relationship between compile-time analysis and runtime behavior, while providing practical guidance for public API design.

Fixes #22704.

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Internal previews

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

[BillWagner]

This LGTM.