Add changelog entry.

CHANGELOG.rst

@@ -5,6 +5,13 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+LMS: Problem rescoring.  Added options on the Grades tab of the
+Instructor Dashboard to allow all students' submissions for a
+particular problem to be rescored.  Also supports resetting all
+students' number of attempts to zero.  Provides a list of background
+tasks that are currently running for the course, and an option to
+see a history of background tasks for a given problem.
+
 LMS: Forums.  Added handling for case where discussion module can get `None` as
 value of lms.start in `lms/djangoapps/django_comment_client/utils.py`
 

lms/djangoapps/instructor_task/api.py

 """
 API for submitting background tasks by an instructor for a course.
 
-TODO:
+Also includes methods for getting information about tasks that have
+already been submitted, filtered either by running state or input
+arguments.
 
 """
 

lms/djangoapps/instructor_task/tasks.py

@@ -2,6 +2,22 @@
 This file contains tasks that are designed to perform background operations on the
 running state of a course.
 
+At present, these tasks all operate on StudentModule objects in one way or another,
+so they share a visitor architecture.  Each task defines an "update function" that
+takes a module_descriptor, a particular StudentModule object, and xmodule_instance_args.
+
+A task may optionally specify a "filter function" that takes a query for StudentModule
+objects, and adds additional filter clauses.
+
+A task also passes through "xmodule_instance_args", that are used to provide
+information to our code that instantiates xmodule instances.
+
+The task definition then calls the traversal function, passing in the three arguments
+above, along with the id value for an InstructorTask object.  The InstructorTask
+object contains a 'task_input' row which is a JSON-encoded dict containing
+a problem URL and optionally a student.  These are used to set up the initial value
+of the query for traversing StudentModule objects.
+
 """
 from celery import task
 from instructor_task.tasks_helper import (update_problem_module_state,