Clarify sizeof rules for unsafe code blocks

[BillWagner]

Some rules regarding nint, nuint, and ref struct types wasn't as clear as it should be. Add examples for nint, nuint, and Span<T>.

Fixes #51281

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/operators/sizeof.md	sizeof operator - determine the memory needs for a given type

[Copilot]

Pull request overview

This pull request clarifies the documentation for the sizeof operator, specifically addressing issue #51281 regarding the rules for using sizeof with different types in safe and unsafe code contexts. The changes improve accuracy by explicitly stating that nint, nuint, and ref struct types like Span<T> require unsafe context, and provide concrete examples demonstrating their usage.

Changes:

• Updated documentation to clarify that safe code requires built-in unmanaged types whose size is not platform-dependent
• Rewrote the unsafe code section with more precise descriptions of how sizeof works with different type categories
• Added code examples demonstrating sizeof with nint, nuint, and Span<int> in unsafe blocks

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
docs/csharp/language-reference/operators/sizeof.md	Updated documentation to clarify platform-dependent type restrictions and unsafe code rules, with improved descriptions of reference, value, and ref struct type behavior
docs/csharp/language-reference/operators/snippets/shared/SizeOfOperator.cs	Added three new examples within the unsafe block demonstrating sizeof usage with nint, nuint, and Span<int>