Add description for System/Mutation Flow Diagrams

System diagram arrows are annotated with numbers and each one has description now.
Mutation Workflow also has description for each step.

Change-Id: I2cf96931cd4d9ac47c93ffb4b7e06adc96c7b314
Reviewed-on: http://review.couchbase.org/34336
Reviewed-by: Sriram Melkote <siri@couchbase.com>
Tested-by: Sriram Melkote <siri@couchbase.com>
Tested-by: Pratap Chakravarthy <pratapsrc@gmail.com>