docs(msbuild): clarify AssemblySearchPaths defaults for SDK-style projects

[fuguiKz]

The AssemblySearchPaths section currently says “ten locations … (four if you use the .NET SDK)” but doesn’t specify which four.

This change:

• Adds a short explanation that AssemblySearchPaths is a semicolon-delimited list of placeholders.
• Lists the default search path order for non-SDK-style projects (numbered list).
• Explicitly lists the 4 default placeholders used by SDK-style projects.

Related: dotnet/msbuild#12949

[prmerger-automator]

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[learn-build-service-prod]

Learn Build status updates of commit 238e69b:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/msbuild/troubleshoot-assembly-references.md	✅Succeeded

For more details, please refer to the build report.

[learn-build-service-prod]

Learn Build status updates of commit 2937bf4:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/msbuild/troubleshoot-assembly-references.md	✅Succeeded

For more details, please refer to the build report.

[Copilot]

Pull request overview

This PR improves the MSBuild documentation by clarifying the default AssemblySearchPaths configuration, specifically addressing the difference between SDK-style and non-SDK-style projects.

Key changes:

• Explicitly lists all 10 default search paths for non-SDK-style projects in a numbered list
• Specifies the 4 default search paths used by SDK-style projects
• Adds explanation that AssemblySearchPaths is a semicolon-delimited list of placeholders that expand during resolution
• Provides guidance on how to inspect the effective search paths using MSBuild tooling

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[v-regandowner]

@ghogen

Can you review the proposed changes?

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#label:"aq-pr-triaged"
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