Update instructions for accounts blocked by PS

Why is this change being made?

After discussing a Zendesk ticket in Slack, it was discovered that the process for unblocking accounts that were blocked as a result of a Professional Services migration is incomplete. This MR explains this scenario in more detail and provides a process for these issues going forward.

Slack convo (for posterity):

@cynthia: Petar: in general, can we unblock users part of a PS migration as long as they ask for it or a group owner requests it?

@pprokic: Hi @cynthia , the intention with leaving the admin note was in case a PS migrated customer needs to unblock users that came back (external, contractors, etc.) since the migration.

@cynthia: Is that a yes then? 😅

@pprokic: Yes 😊 Open for any suggestions how to further improve the process

@cynthia: @mgibsongl : would you be willing to make a quick MR to our workflow page to mention the above? Please assign to Petar for approval, and myself for review

Author Checklist

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