Update each documentation to be inline with reduce

Given that .each uses .reduce under the hood, we should use the same language as .reduce as to the order in which each of the items are called.

https://github.com/petkaantonov/bluebird/blob/master/src/each.js

Without being explicit in the docs it is easy to believe that .each does not wait for the previous promise to resolve before continuing onto the next one.