Update docs for 17.8 upgrades

What does this MR do and why?

Alert users of a possible error that might come up when upgrading to 17.8 related to default work item types created in the work_item_types table.

A user reported the documented problem in #423483 (comment 2385606585) and we had another one through customer support. We are not backporting a fix as we only do bug backports for the current stable release (17.9 at the time of writing). And since 17.8 is a required stop, backporting to 17.9 won't help as everyone has to run the migration in 17.8.

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