Add clarifying comments into shifty Perl code we commented out

utils/pintos

@@ -929,6 +929,9 @@ sub exec_setitimer {
     exit (1);
 }
 
+# This subroutine has been commented out because otherwise if it is not then
+# Pintos will refuse to run at all once the code for putting arguments into
+# the stack is added
 #sub SIGVTALRM {
 #    use Config;
 #    my $i = 0;