Update #each docs for groupedRows

Good news: I added a line that mentions the last undocumented option
that is used in #each helper.

Bad news: I have no idea what the groupedRows boolean is supposed to
accomplish. Something about metamorphs maybe? Also what is a metamorph?

It seems that if `groupedRows` is true or if an `itemViewClass` is
supplied, then it wil use EachView instead of `GroupedEach`. Strange to me
that `groupedRows=true` will result in _not_ using `GroupedEach`.

That is to say that using `GroupedEach` is default and if you supply
either `groupedRows=true` or `itemViewClass=App.FooView`, then it will
use EachView instead.