doc: sort require statements in tests

PR-URL: nodejs#10616
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>

doc/guides/writing-tests.md

@@ -90,6 +90,9 @@ These modules are required for the test to run. Except for special cases, these
 modules should only include core modules.
 The `assert` module is used by most of the tests to check that the assumptions
 for the test are met.
+Note that require statements are sorted, in
+[ASCII](http://man7.org/linux/man-pages/man7/ascii.7.html) order (digits, upper
+case, `_`, lower case).
 
 **Lines 10-21**