#each helper needs better documentation

The [current documentation](http://canjs.com/docs/can.Mustache.helpers.each.html) for the Mustache `{{#each}}` helper doesn't seem to talk about why it is different from the simple Mustache iteration (in that it removes and adds elements to a list dynamically and provides @key and @index directives).

Start with the current Mustache #each documentation at http://canjs.com/docs/can.Mustache.helpers.each.html and compare it with the simple Mustache iteration behavior described elsewhere in the project. Update the documentation to explain dynamic list additions and removals and the @key and @index directives; done means these differences are clearly covered on the page.

Written by the indexing model from the issue text.