docs: improve note about Quadlet TimeoutStartSec

[salim-b]

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Does this PR introduce a user-facing change?

None

[packit-as-a-service]

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[Honny1]

LGTM

[ygalblum]

LGTM

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Honny1, salim-b, ygalblum

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[baude]

lgtm