Provide better documentation for Vitals API

Vitals API documentation provided at https://developer.couchbase.com/documentation/server/current/monitoring/monitoring-n1ql-query.html

is hard to understand. Customers are asking for better documentation with clear description of the statistics it exposes. In particular, they would like to see the following questions answered in the updated documentation.

"gc.num":11607200,                        - What does this number mean? Is it number of KB/MB of garbage before GC kicks in?

"gc.pause.time":"1.849506783s",  - Is this a cap on the number of seconds allowed for a GC pause?

"gc.pause.percent":0,                      - What does this number mean?

"memory.usage":10019216,           - Is this in KB or MB? And is it amount of memory that query engine is currently using?

"memory.total":5628336088,         - Is this in KB or MB?

"memory.system":29055224,          - Is this amount of memory that OS is currently using?

    ....

"request.completed.count":111,     - Is this number of queries that have been served since query engine was started?

"request.per.sec.1min":0.0007,      - What is this number? A percentage? Is it the number of queries/sec in the last minute?

"request.per.sec.5min":0.0107,      - Is this the number of queries/sec in the last 5 minutes?

    ....

"request_time.mean":"170.870877ms",   - Is this mean time for all queries that have been served since query engine was started?