Fix few things related to query_counter context manager:

- Improve doc
- Fix the fact that the context was modifying the initial profiling_level in case it was != 0
- Ignores 'killcursors' to fix flaky test that were impacted by killcursors queries (#1869)

tests/queryset/queryset.py

@@ -4714,18 +4714,27 @@ class QuerySetTest(unittest.TestCase):
         for i in range(100):
             Person(name="No: %s" % i).save()
 
-        with query_counter() as q:
-            self.assertEqual(q, 0)
-            people = Person.objects.no_cache()
+            with query_counter() as q:
+                try:
+                    self.assertEqual(q, 0)
+                    people = Person.objects.no_cache()
 
-            [x for x in people]
-            self.assertEqual(q, 1)
+                    [x for x in people]
+                    self.assertEqual(q, 1)
 
-            list(people)
-            self.assertEqual(q, 2)
+                    list(people)
+                    self.assertEqual(q, 2)
+
+                    people.count()
+                    self.assertEqual(q, 3)
+                except AssertionError as exc:
+                    db = get_db()
+                    msg = ''
+                    for q in list(db.system.profile.find())[-50:]:
+                        msg += str([q['ts'], q['ns'], q.get('query'), q['op']])+'\n'
+                        msg += str(q)
+                    raise AssertionError(str(exc) + '\n'+msg)
 
-            people.count()
-            self.assertEqual(q, 3)
 
     def test_cache_not_cloned(self):
 
@@ -5053,7 +5062,7 @@ class QuerySetTest(unittest.TestCase):
                                            {"$ne": "%s.system.indexes" % q.db.name}})[0]
 
             self.assertFalse('$orderby' in op['query'],
-                             'BaseQuerySet must remove orderby from meta in boolen test')
+                             'BaseQuerySet must remove orderby from meta in boolean test')
 
             self.assertEqual(Person.objects.first().name, 'A')
             self.assertTrue(Person.objects._has_data(),