Refine wording around SoftPCL deployments and Post-deployment migrations

Why is this change being made?

@sloyd @alanrichards @amyphillips I reworded this to be more explicit. The previous change was reading like some deployments are containing post-deploy migrations and those are executed automatically when the deployment happens. This would cause confusion and doubts (due to the increased risk) on the EOC.

Instead, Post-deploy migrations are a manual step, eventually triggered, after a deployment and are fully intentional by the Release Manager. This wording hopefully makes this clearer for the EOCs.

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