Improve Pinpoint notification docs

[palpatim]

From PR #82, @undefobj commented:

I think this is good however one thing I would like to add in the next pass is a "How it works" for the Pinpoint Analytics stuff. For instance in the Kinesis section we talk about why we stream to disk with a PutRecord but we don't talk about the same in Pinpoint. We do talk about managing sessions but we don't really talk about why they're useful aside from "Indicate when and how often users open and close your app" which isn't completely accurate as sessions can be impacted by backgrounding, or another app (like a text message) popping up. Also for both the Pinpoint and Kinesis stuff if we are batching to disk, when does the record actually get sent up to the service? Is it when network connectivity resumes or at another scheduled time?

[mutablealligator]

I am trying summarize the ask from @undefobj here. Add explanation for the following questions:

1. How does batching work when recordEvent is called? What is the condition that needs to be met for the events to be sent to Pinpoint?

2. How is session management useful and how does it work?