update rubric criteria api docs

Some properties were missing

Test Plan:
 * RubricCriteria object description should have outcome_id/vendor_guid/long_description

closes PLAT-811

Change-Id: I5a5d264c49a4d7ad6406e2574b5c3e6c69683ee9
Reviewed-on: https://gerrit.instructure.com/46196
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>

app/controllers/assignments_api_controller.rb

@@ -105,13 +105,28 @@
 #           "type": "integer"
 #         },
 #         "id": {
+#           "description": "The id of rubric criteria.",
 #           "example": "crit1",
 #           "type": "string"
 #         },
+#         "outcome_id": {
+#           "description": "(Optional) The id of the learning outcome this criteria uses, if any.",
+#           "example": "1234",
+#           "type": "string"
+#         },
+#         "vendor_guid": {
+#           "description": "(Optional) The 3rd party vendor's GUID for the outcome this criteria references, if any.",
+#           "example": "abdsfjasdfne3jsdfn2",
+#           "type": "string"
+#         },
 #         "description": {
 #           "example": "Criterion 1",
 #           "type": "string"
 #         },
+#         "long_description": {
+#           "example": "Criterion 1 more details",
+#           "type": "string"
+#         },
 #         "ratings": {
 #           "type": "array",
 #           "items": { "$ref": "RubricRating" }