Update Debugger.xml (#4704)

Author: danmoseley

xml/System.Diagnostics/Debugger.xml

@@ -356,6 +356,8 @@ Console.WriteLine("Hello, world.");
   
 ## Remarks  
  If a debugger is already attached, nothing happens.  
+
+Debugger launch is only supported on Windows. On Unix operating systems, the method returns `true` without launching a debugger.
   
  ]]></format>
         </remarks>