Update docs for async_status .started and .finished properties

[oraNod]

Follows on from ansible/ansible#85048

Need to update docs for the .started and .finished properties with the ansible.builtin.async_status module as @evgeni pointed out in the issue.

Fix:

• Update example at: https://github.com/ansible/ansible/blob/0aa8afbaf4739510a96c9727237792a95c8855c3/lib/ansible/modules/async_status.py#L67
• Add an entry to the 2.19 porting guide: https://github.com/ansible/ansible-documentation/blob/devel/docs/docsite/rst/porting_guides/porting_guide_core_2.19.rst

[samccann]

ansible/ansible#85066 fixes the item in ansible/ansible

[oraNod]

#2576 fixes the porting guide