snf_django: Explain why @with_settings needs wraps

Add comment about why the use of functools.wraps is mandatory for the
decorators that are used in test functions.

snf-django-lib/snf_django/utils/testing.py

@@ -124,6 +124,9 @@ def override_settings(settings, **kwargs):
 
 def with_settings(settings, prefix='', **override):
     def wrapper(func):
+        # The use of wraps is required here. Without it, the name of a "test_"
+        # function that gets decorated by "with_settings" would become "inner"
+        # and nose would not detect it as a test function.
         @functools.wraps(func)
         def inner(*args, **kwargs):
             with override_settings(settings, prefix=prefix, **override):