I've never seen any official Tableau documentation, but Interworks seems to know how the XML is put together. At times I've even played with the XML as well (there is no easy way to create a 10,000 colour legend ), but I think if Tableau started documenting it then it would slow down their version releases as well
dave holmes-kinsella check out Power Tools for Tableau created by InterWorks. It's a toolkit we originally created for internal use but along the way decided everyone might have a use for parts of it.
Specifically, there's a 14 day free trial of Workbook Tools that has an Audit feature that will export a csv with the information you're looking for. I'd check out the Workbook SDK while you're there as it might be right up your alley.