Add comment indicating that tools shall not be exported

Author: Planeshifter

lib/node_modules/@stdlib/error/lib/index.js

@@ -22,6 +22,10 @@
 * When adding modules to the namespace, ensure that they are added in alphabetical order according to module name.
 */
 
+/*
+* The following modules are intentionally not exported: tools
+*/
+
 // MODULES //
 
 var setReadOnly = require( '@stdlib/utils/define-read-only-property' );