Add example for __test__ (#7733)

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

doc/en/example/pythoncollection.rst

@@ -313,3 +313,12 @@ interpreter:
     collect_ignore = ["setup.py"]
     if sys.version_info[0] > 2:
         collect_ignore_glob = ["*_py2.py"]
+
+Since Pytest 2.6, users can prevent pytest from discovering classes that start
+with ``Test`` by setting a boolean ``__test__`` attribute to ``False``.
+
+.. code-block:: python
+
+    # Will not be discovered as a test
+    class TestClass:
+        __test__ = False