Tinker with the wording of the short_open_tag description.

I'm still not thrilled with this, but it's a bit better and addresses the
issues noted in bug #65644. Plus, it now mentions the decoupling of <?=.
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  Tinker with the wording of the short_open_tag description.