Document the new "Runtime" metadata in UsingTask

[ViktorHofer]

Also clearly call out that this new TaskHost feature is currently only supported when using the "Microsoft.NET.Sdk" msbuild sdk.

Related: #12895

[baronfel]

We need to update both https://github.com/MicrosoftDocs/visualstudio-docs/blob/main/docs/msbuild/usingtask-element-msbuild.md for the 'raw' docs about what values are allowed in the UsingTask, but we should also update https://github.com/MicrosoftDocs/visualstudio-docs/blob/main/docs/msbuild/how-to-configure-targets-and-tasks.md with more conceptual-level documentation about

• Task Hosts in general
  • what they do (run tasks)
  • why the InProc and OutOfProc Task Hosts exist
    • in-proc for efficiency
    • out-of-proc for compatibility and cross-runtime support
  • how perf of out of proc is mitigated by the engine
    • pooling the TaskHost nodes during the duration of the build
• the new .NET TaskHost feature
  • how the detection occurs (Runtime="NET" when running on a Framework MSBuild instance)
  • the perf implications of executing in this mode
    • slight overhead due to sending messages to the out-of-proc node
    • more expensive the more Log messages a Task emits
    • weigh the maintenance benefit vs the potential perf (negative) of being out of proc vs the potential perf (positive) of running on the more efficient .NET runtime
  • limitations today
    • only usable for project types that use the Microsoft.NET.Sdk MSBuild SDK

[ViktorHofer]

The feature already shipped with .NET 10 / VS 18.0, right? If so, then we should document this the sooner the better.

[baronfel]

Working on a copilot PR to the docs repo as we speak

[baronfel]

PR raised on internal docs repo.