Specify a wait time to clarify "immediate" deletion

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What does this MR do?

Replaces mention of "immediate" deletion by specifying a time window for completing the deletion. In a recent ticket, deletion completed in 40 minutes, during which time the user was blocked from creating a new account. A block on new account creation was unexpected since our docs say deletion is "immediate".

This MR seeks to clarify that there may be a delay in immediate deletion so that the user knows to wait before creating a new account and/or has an explanation for the block behaviour should it occur.

Related issues

• Customer ticket

Logs showing that the deletion process takes ~40 minutes to complete:

Rails logs proof of deletion delay for "immediate deletion":

Sidekiq log | Registration was only possible once Users::MigrateRecordsToGhostUserInBatchesWorker completed:

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Reviewer's checklist

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