generate.pl pod update; add desc of test_parallel

Author: PeterMartini

generate.pl

@@ -117,6 +117,12 @@ =head1 DESCRIPTION
 Additional text to pass to C<Configure>.  At the moment, this is necessary for
 5.18.x so that it can get the C<-fwrapv> flag.
 
+=item (optionally) test_parallel
+
+This can be either 'no', 'yes', or unspecified (equivalent to 'yes').
+Added due to dist/IO/t/io_unix.t failing when TEST_JOBS > 1, but should
+only be used in case of a documented issue.
+
 =back
 
 =cut