Fix usage of {curves,key-exchanges}.pl in all.sh

tests/scripts/curves.pl

@@ -17,9 +17,12 @@
 #
 # And any test suite with the wrong dependencies will fail.
 #
-# Usage: curves.pl
+# Usage: tests/scripts/curves.pl
 #
 # This script should be executed from the root of the project directory.
+#
+# For best effect, run either with cmake disabled, or cmake enabled in a mode
+# that includes -Werror.
 
 use warnings;
 use strict;