Clarify migration upgrade doc for 15.9

Max Oreficerequested to merge
morefice/update-migration-doc-15.9 into master

What does this MR do and why?

This MR clarifies the upgrade doc for %15.9 and avoid the confusion.

As pointed out by @bbodenmiller the upgrade path for self-instances is different and migrations are performed after upgrading the software.

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