[msbuild] Explain WriteLinesToFile copy behavior

Document why copying File back to Windows does not require suppressing a remoting output placeholder.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: e9d84281-7deb-4b10-9c48-282faafc8f34

Author: rolfbjarne

msbuild/Xamarin.MacDev.Tasks/MsBuildTasks/WriteLinesToFile.cs

@@ -14,6 +14,7 @@ public override bool Execute ()
 				if (!XamarinTask.ExecuteRemotely (this, out var taskRunner))
 					return false;
 
+				// File isn't an [Output] property, so no empty placeholder is created on Windows.
 				if (CopyToWindows)
 					XamarinTask.CopyFilesToWindowsAsync (this, taskRunner, new [] { File }).Wait ();