Improve CQL backend documentation

None currently exists in the Cassandra chapter. I tried enabling it in janusgraph-cassandra-es.properties but got a version mismatch error.

- What is it compatible with?
- Why would it be used? pros/cons?

Start with the Cassandra chapter and the janusgraph-cassandra-es.properties configuration mentioned in the report. Investigate the version mismatch encountered when enabling the CQL backend, then document its compatibility, use cases, advantages, and disadvantages. Done means the Cassandra documentation answers these questions and explains the reported configuration issue.

Written by the indexing model from the issue text.