Docfix: running background tasks without polling

Fix the example in the doc to explicitly set the poll interval to zero so the
job doesn't poll.

To run a background task without polling, you need to set the poll interval to
zero. However, Ansible's default poll setting is 15 seconds, so not specifying the poll
interval will cause a background job to poll.
Merge pull request #9737 from lorin/doc-nopoll

Docfix: running background tasks without polling