Document the intention behind the 'percentage' on adjustments

[rszrama]

We document that adjustments have both an amount and a percentage, but we don't really discuss the relationship between those. Best I can tell, the percentage field is purely informational. I couldn't find anywhere in the code where it was actually used. If this is the case, we should make that clear so folks don't set the percentage and expect that to have some sort of recalculation impact on the amount.

[lisastreeter]

Good idea. Pull request has been created to update the Adjustments documentation page: #474