[docs] [issue28755] Rework syntax highlighing in howto/clinic.rst

Serhiy Storchaka added the comment:

(Ned asked for only release-critical changes to go into 3.6 at the moment.)

Except the documentation changes. ---------- nosy: +serhiy.storchaka _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue28755> _______________________________________