Document serial nature of .each more explicitly

In issue #134, I learned more about different ways to achieve something like ``mapSeries`` with bluebird.  I had previously read the docs for ``.each``, but it wasn't clear to me from the docs that it operated serially.  Just add a quick note to make that clear for any others like me who were searching for some serial method.
Merge pull request #385 from yourcelf/patch-1

Document serial nature of .each more explicitly