We already support fee = fee_rate * weight and also fee rate = fee / weight so it seems logical to support div by fee rate.

Note that division results in a threshold. For fee / weight we want the ceiling because floor would result in too small a fee rate. Following similar logic, for fee / fee rate I believe we want floor otherwise a larger weight would be too big resulting in a higher fee.

If #3813 merges then when doing this issue remember to update the new module rustdoc.