Document Exponential Backoff and Retry Semantics

The current examples in the docs for performing exponential backoffs and retries with the 2.0 SDK could be improved to make it easier for customers, such that they have a more ready made sample to work from.

A couple of example scenarios which are very common for customers:
o Issue an async write and retry a maximum of N times with an exponential back off
o Issue an async read and perform a maximum of N retries from replicas with exponential back off

Some of the individual parts of this are shown... but there isn't an overriding example that combines these together and shows you how to do these very common tasks with Couchbase – quite a few of the examples are based on counting numbers etc.

Also, the examples we do have for performing backoffs are all in Java 8 style, which is still some way off for most customers.