Task: Document that retry(max_retries=None) means "use the default". Closes #1875

celery/app/task.py

@@ -506,6 +506,9 @@ class Task(object):
         :keyword eta: Explicit time and date to run the retry at
                       (must be a :class:`~datetime.datetime` instance).
         :keyword max_retries: If set, overrides the default retry limit.
+            A value of :const:`None`, means "use the default", so if you want infinite
+            retries you would have to set the :attr:`max_retries` attribute of the
+            task to :const:`None` first.
         :keyword time_limit: If set, overrides the default time limit.
         :keyword soft_time_limit: If set, overrides the default soft
                                   time limit.