Add comment explaining ERROR_ACCESS_DENIED handling in TerminateProcessCore

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/1a601c8b-3c5f-4596-9b8f-5e11188e35ae

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Author: Copilot

src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.Windows.cs

@@ -753,6 +753,7 @@ private bool TerminateProcessCore()
 
             int error = Marshal.GetLastPInvokeError();
 
+            // If the process has already exited, TerminateProcess fails with ERROR_ACCESS_DENIED.
             if (error == Interop.Errors.ERROR_ACCESS_DENIED &&
                 Interop.Kernel32.GetExitCodeProcess(this, out int exitCode) &&
                 exitCode != Interop.Kernel32.HandleOptions.STILL_ACTIVE)