Document PublishRuntimeIdentifier in the .NET SDK MSBuild property reference

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The MSBuild property reference listed publish-time properties but didn't document PublishRuntimeIdentifier, a publish-scoped alternative to RuntimeIdentifier. This update adds that missing guidance and clarifies how the property behaves during publish.

• Docs update

  • Added PublishRuntimeIdentifier to the publish-related property index in docs/core/project-sdk/msbuild-props.md.
  • Added a dedicated section describing it as a publish-only RID property.

• Behavior clarified

  • Notes that the property was introduced in .NET 7.
  • Explains that it doesn't affect build operations.
  • Clarifies that during publish, PublishRuntimeIdentifier sets RuntimeIdentifier.
  • Notes that a globally passed RuntimeIdentifier still takes precedence.

• Usage example

  • Added an XML example showing how to set the property in a project file.

<PropertyGroup>
  <PublishRuntimeIdentifier>linux-x64</PublishRuntimeIdentifier>
</PropertyGroup>

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Internal previews

📄 File	🔗 Preview link
docs/core/project-sdk/msbuild-props.md	docs/core/project-sdk/msbuild-props

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Pull request overview

This PR updates the .NET SDK MSBuild property reference to document PublishRuntimeIdentifier as a publish-scoped alternative to RuntimeIdentifier, including a short description of publish-time behavior and a project-file example.

Changes:

• Added PublishRuntimeIdentifier to the publish-related property index.
• Added a new PublishRuntimeIdentifier section explaining publish-only behavior, precedence, and the .NET version it was introduced in.
• Added an XML example showing how to set PublishRuntimeIdentifier in a project file.