added docstrign to inection collection test.

testing/python/collect.py

@@ -1079,6 +1079,12 @@ def test_dont_collect_non_function_callable(testdir):
 
 
 def test_class_injection_does_not_break_collection(testdir):
+    """Tests whether injection during collection time will terminate testing.
+
+    In this case the error should not occur if the TestClass itself
+    is modified during collection time, and the original method list
+    is still used for collection.
+    """
     testdir.makeconftest("""
         from test_inject import TestClass
         def pytest_generate_tests(metafunc):