add last attended date parameter to API doc

Add documentation for last attended date parameter for last_attended
endpoint of the enrollments API.

closes FOO-3482
flag=none

test plan:
- generate API documentation
- go to enrollments API documentation
- make sure add last attended date endpoint has date parameter

Change-Id: Iccca9fbb1a72a6329073b20e19584b03d05d6bbf
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/318174
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Ajmal Esa <ajmal.esa@instructure.com>
Product-Review: Ajmal Esa <ajmal.esa@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>

app/controllers/enrollments_api_controller.rb

@@ -870,13 +870,16 @@ class EnrollmentsApiController < ApplicationController
     end
   end
 
-  # @API Adds last attended date to student enrollment in course
+  # @API Add last attended date
+  # Add last attended date to student enrollment in course
+  #
+  # @argument date [Date]
+  #   The last attended date of a student enrollment in a course.
   #
   # @example_request
   #   curl https://<canvas>/api/v1/courses/:course_id/user/:user_id/last_attended"
   #     -X PUT => date="Thu%20Dec%2021%202017%2000:00:00%20GMT-0700%20(MST)
   #
-  #
   # @returns Enrollment
   def last_attended
     return unless authorized_action(@context, @current_user, [:view_all_grades, :manage_grades])