Staff grading doc updates

Updating a user-facing string to include staff grading. Also updating
some dangerously outdated documentation in code that was unearthed
during PR review.

openassessment/templates/openassessmentblock/edit/oa_edit.html

@@ -157,9 +157,9 @@
 
             <p class="openassessment_description" id="openassessment_step_select_description">
                 {% if 'example_based_assessment' in editor_assessments_order %}
-                    {% trans "In this assignment, you can include steps for learner training, peer assessment, self assessment, and example based assessment. Select the steps that you want below, and then drag them into the order that you want. If you include an example based assessment step, it must precede all other steps. If you include a learner training step, it must precede peer and self assessment steps." %}
+                    {% trans "In this assignment, you can include learner training, peer assessment, self assessment, example based assessment, and staff assessment steps. Select the steps that you want below, and then drag them into the order you want. If you include an example based assessment step, it must precede all other steps. If you include a learner training step, it must precede peer and self assessment steps. If you include a staff assessment step, it must be the final step. After you publish an ORA assignment, you cannot change the type and number of assessment steps." %}
                 {% else %}
-                    {% trans "In this assignment, you can include steps for learner training, peer assessment, and self assessment. Select the steps that you want below, and then drag them into the order that you want. If you include a learner training step, it must precede all other steps." %}
+                    {% trans "In this assignment, you can include learner training, peer assessment, self assessment, and staff assessment steps. Select the steps that you want below, and then drag them into the order you want. If you include a learner training step, it must precede all other steps. If you include a staff assessment step, it must be the final step. After you publish an ORA assignment, you cannot change the type and number of assessment steps." %}
                 {% endif %}
             </p>
             <ol id="openassessment_assessment_module_settings_editors">

openassessment/xblock/validation.py

@@ -96,11 +96,9 @@ def _is_valid_assessment_sequence(assessments):
 
 def validate_assessments(assessments, current_assessments, is_released, _):
     """
-    Check that the assessment dict is semantically valid.
-
-    Valid assessment steps are currently:
-    * peer, then self
-    * self only
+    Check that the assessment dict is semantically valid. See _is_valid_assessment_sequence()
+    above for a description of valid assessment sequences. In addition, enforces validation
+    of several assessment-specific settings.
 
     If a question has been released, the type and number of assessment steps
     cannot be changed.