Best way to document database mappings for migrations
The most common way to do this Excel, but it has it's tradeoffs:


Very difficult to update the spreadsheet if either side of the migration changes (due to missing something, something changing, etc.)
Excel is harder to use by the people implement the rules documented in it.
Change tracking is nearly impossible if you have multiple people collaborating on the document.


I prefer to use data lineage features of data modelling tools such as ER/Studio, ERwin or PowerDesigner to track the transformations required for migration from one data store to another.  I get all the benefits of RE and compares to the source and target, plus none of the tradeoffs mentioned above.  It really makes a difference.