Update DebugType documentation to clarify default behavior and show how to disable PDB generation for Release builds

[Copilot]

The DebugType documentation was missing crucial information about default values and how to configure different behavior for Debug vs Release builds, leading to user confusion about PDB file generation.

Changes Made

1. Clarified Default Behavior

• Updated the description to explicitly state that the default value is portable for both Debug and Release configurations
• Removed ambiguous language that didn't specify actual default values
• Made it clear that PDB files are generated by default for all configurations in modern .NET projects

2. Added Missing Documentation

• Added the none value to the DebugType values table, which was previously missing
• This value is essential for users who want to disable PDB generation entirely

3. Added Practical Guidance Section

Created a new "Disabling PDB generation for Release builds" section with working examples:

<!-- Primary recommended approach -->
<PropertyGroup Condition="'$(Configuration)' == 'Release'">
  <DebugType>none</DebugType>
</PropertyGroup>

<!-- Alternative approach -->
<PropertyGroup Condition="'$(Configuration)' == 'Release'">
  <DebugSymbols>false</DebugSymbols>
</PropertyGroup>

4. Referenced Breaking Changes

• Added a note about .NET 8 behavior changes regarding DebugSymbols
• Explained why DebugType=none is the most reliable approach

Verification

• Tested both approaches with .NET 8 projects
• Confirmed that DebugType=none reliably prevents PDB generation for Release builds
• Verified that Debug builds continue to generate PDB files as expected

This directly addresses the user's question: "How do I get .PDB files only for Debug output only?" and provides accurate information about default behavior.

Fixes #27071.

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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]

@gewarren It looks like copilot took all your suggestions. Can you give it another look?