Update documentation to reflect multiple Redis support

Summary

Work for #1644 (closed) introduced the ability to have gitlab/gitlab consume different external Redis instances for different storage types.

We now need to document these changes, and the impact of enabling these features.

Details

Split from #1644 (closed) & !1287 (merged)

Documentation to be done:

• Add additional Redis properties to documentation
• Move Redis configuration documentation out of Unicorn
• Add documentation of multiple Redis to doc/advanced/external-redis