Clarified CI Job Token auth. caveats

See issue #1620

docs/api-usage.rst

@@ -29,14 +29,16 @@ To connect to GitLab.com or another GitLab instance, create a ``gitlab.Gitlab``
    gl = gitlab.Gitlab('https://gitlab.example.com', oauth_token='my_long_token_here')
 
    # job token authentication (to be used in CI)
+   # bear in mind the limitations of the API endpoints it supports:
+   # https://docs.gitlab.com/ee/ci/jobs/ci_job_token.html
    import os
    gl = gitlab.Gitlab('https://gitlab.example.com', job_token=os.environ['CI_JOB_TOKEN'])
 
    # Define your own custom user agent for requests
    gl = gitlab.Gitlab('https://gitlab.example.com', user_agent='my-package/1.0.0')
 
    # make an API request to create the gl.user object. This is mandatory if you
-   # use the username/password authentication.
+   # use the username/password authentication (not required for job token authentication).
    gl.auth()
 
 You can also use configuration files to create ``gitlab.Gitlab`` objects: