Add grades how_tos doc.

lms/djangoapps/grades/docs/how_tos/README.rst

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+How-to Guides for Grades
+========================
+
+`How to re-compute grades for all learners in a course <how-to-regrade-everyone-in-a-course.rst>`_

lms/djangoapps/grades/docs/how_tos/how-to-regrade-everyone-in-a-course.rst

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+How to Update Grades for Every Learner in a Course
+--------------------------------------------------
+
+This will help if you want to re-computer the persistent subsection and course
+grades for every learner in a course.
+**If grades for your course are currently frozen, you must first complete the "What to do if grades are frozen" section below.**
+
+Steps to trigger course-wide re-grade
+=====================================
+
+1. Go to the Studio grading settings for the course,
+   for example https://studio.stage.edx.org/settings/grading/edX/DemoX/Demo_Course .
+
+2. Make a trivial edit to one of the assignment type names.  For example,
+   for an assignment type named "Homework", add a space to get "Homework ",
+   and then delete the space.
+
+3. Studio should now alert you that you've made changes;
+   click the "Save Changes" button at the bottom of your screen.
+
+4. Changing the grading policy of the course (even though you made no change of consequence)
+   will cause an asynchronous task called ``compute_all_grades_for_course`` to be enqueued.
+   This task re-computes grades for every graded subsection in the course, for every learner
+   in the course.  This will also cause course grades to be re-computed for
+   every learner in the course.  Note that, for the purpose of "throttling",
+   **there's a 1 hour delay between the enqueueing of this task and the actual
+   re-computation of grades.**
+
+5. Exit Studio.
+
+6. Ask an engineer to help you with the "Monitoring" section below.
+
+What to do if grades are frozen
+===============================
+
+By default, we don't allow for subsection or course grades to be changed after
+30 days from the end date of a course have elapsed.  If the course you want to
+update grades for currently has "frozen" grades, you need to do the following:
+
+1. Go to the Django Admin page and find the "Waffle flag course overrides" page, for example
+   https://courses-internal.stage.edx.org/admin/waffle_utils/waffleflagcourseoverridemodel/
+
+2. Ensure that there is not currently an entry with the flag name
+   ``grades.enforce_freeze_grade_after_course_end`` for this course.
+
+3. Click the "Add Waffle Flag Course Override" button.
+
+4. Enter the flag name from step (2) in the "Waffle flag" field.
+
+5. Enter the course id of the course.
+
+6. Select "Force Off" for the "Override choice" field.
+
+7. Check the "Enabled" checkbox.
+
+8. Click "Save"
+
+9. Exit the Django Admin site and proceed to the section above.
+
+Monitoring
+==========
+
+You should be able to track the progress of the re-grade by searching
+for your course id and the ``compute_all_grades_for_course`` task name in Splunk.
+Initially, that task will have been received, but not started.  When it does start,
+you'll see a bunch of child tasks like ``recalculate_course_and_subsection_grades_for_user``
+that indicate grades are being re-calculated for the learners in this course.