CI Partitioning - Document long running migration

Max Oreficerequested to merge
morefice/document-long-running-migration into master

Ref: #390480 (closed)

What does this MR do and why?

As brought up in !107547 (comment 1258716885).

This MR documents a long running migration introduced for CI Partitioning.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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