simplify comment and code
simplify comment and code

the comment was mostly describing invariants for replacing group.top()
with group.all() + computeOrderedGroups + slice (9d07086cad)

slice's second parameter has a reasonable default
for #1184

aaronhoffman/dc.js,mtraynham/dc.js,TheDataShed/dc.js,dc-js/dc.js,dc-js/dc.js,mtraynham/dc.js,TheDataShed/dc.js,mtraynham/dc.js,aaronhoffman/dc.js,dc-js/dc.js,TheDataShed/dc.js,aaronhoffman/dc.js,aaronhoffman/dc.js,aaronhoffman/dc.js,dc-js/dc.js,dc-js/dc.js,TheDataShed/dc.js,TheDataShed/dc.js,mtraynham/dc.js,mtraynham/dc.js