add jsonapi pagination to polling and missing documentation

fixes CNVS-13534
This commit adds 2 things.  One is that, when it's requested via the
'Accept' request header with a value of 'application/vnd.api+json', it
will return jsonapi standard pagination.  The other is the addition of
of documentation for the 'results' attribute on poll sessions, and the
'total_results' attribute on polls.

Test plan
1. jsonapi pagination
- As a teacher, and using a header of Accept: application/vnd.api+json
  Test the following endpoints:
  - The polls index endpoint
  - The poll_choices index endpoint
  - The poll_sessions index endpoint
  - The poll_sessions opened endpoint
  - The poll_sessions closed endpoint
  - Each of these endpoints should return the same JSON response, but
    with the addition of a 'meta' attribute that has a 'pagination' key,
    with pagination information.

2. documentation
- The documentation for polls should now include a 'total_results'
  attribute, and the documentation for poll_sessions should include a
  'results' attribute.

Change-Id: I2a16b4d32b3b2a4f2023628b24f9accaecbffcc1
Reviewed-on: https://gerrit.instructure.com/36216
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>