Clarify TargetFramework as TFM alias and document SDK 10.0.300 multi-TFM equivalence

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The TargetFramework/TargetFrameworks MSBuild property docs didn't explain that these values are aliases the SDK translates into canonical moniker properties, nor the SDK 10.0.300 behavior where multiple TargetFrameworks values can resolve to the same effective framework.

Changes to msbuild-props.md

• TargetFramework: Adds a note that the value is an alias — the .NET SDK parses it and sets TargetFrameworkMoniker, TargetFrameworkIdentifier, TargetFrameworkVersion, and (when applicable) TargetPlatformIdentifier, TargetPlatformVersion, and TargetPlatformMoniker. Notes that users can set these properties directly when using a custom alias.

• TargetFrameworks: Adds a note that starting with .NET SDK 10.0.300, multiple values can resolve to the same effective framework — for example:

  <TargetFrameworks>net10.0-linux;net10.0-mac</TargetFrameworks>

  Both net10.0-linux and net10.0-mac resolve to net10.0.

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Internal previews

📄 File	🔗 Preview link
docs/core/project-sdk/msbuild-props.md	MSBuild reference for .NET SDK projects

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Pull request overview

This PR updates the .NET SDK MSBuild property reference to clarify how TargetFramework/TargetFrameworks values are interpreted, including noting that the SDK parses TFMs into other derived MSBuild properties, and documenting a .NET SDK 10.0.300 multi-targeting equivalence behavior.

Changes:

• Added a note under TargetFramework explaining that the SDK parses the value and populates related MSBuild properties.
• Added a note under TargetFrameworks about .NET SDK 10.0.300 allowing multiple entries that resolve to the same effective framework.