Add docs for task_manager and tests/manager/utils.
Add docs for task_manager and tests/manager/utils.

Change-Id: Ib740f3263de3676fb1ab393746a7a6a9161f536b

faizan-barmawer/elytics,faizan-barmawer/ironic-lib,citrix-openstack-build/ironic-lib,NaohiroTamura/ironic-lib,openstack/ironic-lib