Include bullet points for unavailable and request error in DefaultRetryPolicy class top level javadoc

Description

From mailing list: https://groups.google.com/a/lists.datastax.com/forum/#!topic/java-driver-user/0io6HfftEuo

This policy retries queries in only two cases:

On a read timeout, if enough replicas replied but data was not retrieved.
On a write timeout, if we timeout while writing the distributed log used by batch statements.

This does not include detail about unavailable and request error cases, we should include those.

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Assignee

Andy Tolbert

Reporter

Andy Tolbert

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