[DebugInfo][RemoveDIs] Add documentation for updating code to handle debug records (#93562)

Although the patch that enables debug records by default has been
temporarily reverted, it will (eventually) be reverted and everyone's
code will be subjected to the new debug info format. Although this is
broadly a good thing, it is important that the documentation has enough
information to guide users through the update; this patch adds what
should hopefully be enough detail for most users to either find the
answers, or find out how to find those answers.