Improved strip_tags and clarified documentation

The fact that strip_tags cannot guarantee to really strip all
non-safe HTML content was not clear enough. Also see:
https://www.djangoproject.com/weblog/2014/mar/22/strip-tags-advisory/

tests/utils_tests/test_html.py

@@ -80,6 +80,8 @@ class TestUtilsHtml(TestCase):
             ('a<p a >b</p>c', 'abc'),
             ('d<a:b c:d>e</p>f', 'def'),
             ('<strong>foo</strong><a href="http://example.com">bar</a>', 'foobar'),
+            ('<sc<!-- -->ript>test<<!-- -->/script>', 'test'),
+            ('<script>alert()</script>&h', 'alert()&h'),
         )
         for value, output in items:
             self.check_output(f, value, output)