Fix Deterministic compiler option default value documentation

[Copilot]

The documentation for the Deterministic C# compiler option was misleading about the default behavior. The previous text stated "By default, compiler output from a given set of inputs is unique, since the compiler adds a timestamp and an MVID..." which implied that deterministic compilation was disabled by default.

However, for modern .NET projects (including MAUI projects as mentioned in the issue), the .NET SDK actually sets Deterministic=true by default. This was causing confusion for developers who expected the default to be false based on the documentation.

Changes Made

• Explicitly state the default value: Added clear language that deterministic compilation is enabled by default for modern .NET projects (Deterministic property defaults to true)
• Clarify when deterministic vs non-deterministic compilation occurs: Separated the explanation to clearly distinguish between the default behavior (deterministic) and what happens when explicitly disabled
• Maintain technical accuracy: Preserved all the detailed information about what inputs affect determinism while restructuring for clarity

Validation

Tested with actual .NET projects to confirm:

• Default behavior: Deterministic = true (confirmed in build output)
• Explicit override: <Deterministic>false</Deterministic> works as expected
• Multi-target projects (MAUI-style): Also default to true

This ensures the documentation accurately reflects the actual SDK behavior that developers experience.

Fixes #33380.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-options/code-generation.md	docs/csharp/language-reference/compiler-options/code-generation

[BillWagner]

This is ready for final review.

[BillWagner]

@baronfel Do you want to give this one a look?

[baronfel]

This LGTM!