Clarify order of signal handler execution for .NET 10 (#11439)

Follow up on dotnet/runtime#116557

Author: jkotas

xml/System.Runtime.InteropServices/PosixSignalRegistration.xml

@@ -82,6 +82,7 @@ Raw values can be provided for `signal` on Unix by casting them to <xref:System.
             <xref:System.Runtime.InteropServices.PosixSignal.SIGINT> and <xref:System.Runtime.InteropServices.PosixSignal.SIGQUIT> can be canceled on both
             Windows and on Unix platforms; <xref:System.Runtime.InteropServices.PosixSignal.SIGTERM> can only be canceled on Unix.
             On Unix, terminal configuration can be canceled for <xref:System.Runtime.InteropServices.PosixSignal.SIGCHLD> and <xref:System.Runtime.InteropServices.PosixSignal.SIGCONT>.
+            In .NET 10 and later versions, the handlers are executed in reverse order of their registration.
 
           ]]></format>
         </remarks>