Clarify behavior of placement_group_capture_child_tasks in docs

[amogkam]

Explicitly specify that placement_group_capture_child_tasks value needs to be set for every actor and the value is not inherited from its parent actor.

Why are these changes needed?

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[rkooo567]

cc @angelinalg

[rkooo567]

doc build failure @amogkam

[can-anyscale]

please rebase, the readsthedoc build is fixed on lastest master, thankks

[angelinalg]

just some nits