Document NullifyOrphanRunnerIdOnCiBuilds migration failure

What does this MR do and why?

Related to #415724 (closed)

In %14.9 we've added a background migration(that was not completed) and in %15.10 we've changed the database primary key for the table which prevents ActiveRecord from generating a valid SQL query. If someone upgrades from 14.9 to 15.11(which is the hard stop to complete all background migrations) this migration could fail.

More details on the events timeline: #415724 (comment 1447201715)

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