document that we have an attachment on account reports

Change-Id: I109366e608467f1272ddbe6796925ac560561b67
Reviewed-on: https://gerrit.instructure.com/116525
Tested-by: Jenkins
Reviewed-by: Brent Burgoyne <bburgoyne@instructure.com>
QA-Review: Tucker McKnight <tmcknight@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>

app/controllers/account_reports_controller.rb

@@ -39,6 +39,9 @@
 #           "example": "https://example.com/some/path",
 #           "type": "string"
 #         },
+#         // The attachment api object of the report. Only available after the report has completed.
+#         // Abbreviated attachment object File (see files API).
+#         "attachment": {},
 #         "status": {
 #           "description": "The status of the report",
 #           "example": "complete",