Sync Gateway _expvar Documentation

The /_expvar endpoint is documented in https://developer.couchbase.com/documentation/mobile/2.0/references/sync-gateway/admin-rest-api/index.html#/server/get__expvar per the sync_gateway repo wiki https://github.com/couchbase/sync_gateway/wiki/expvars.

It is accurate but the documentation should spell out which metrics are important and how to interpret results. Scenarios:

1. "How much channel cache is being used right now?"
2. "What is the Channel Cache and Rev Cache miss ratio?"
3. "Do I need to give Sync Gateway more Channel Cache?"
4. "How many replications (i.e /{db}/_changes connections) is Sync Gateway managing right now?"
5. "Which Golang processes are consuming memory , CPU or Network bandwidth?"
6. "What does it mean when xyz Golang processes are consuming memory, CPU or Network bandwidth?"
7. "What is the average latency of processing a record?"

NB: Fujio Turner created a tool to plot the various metrics returned by _expvar (https://github.com/Fujio-Turner/sg-stats-demo). It's good to know about in the context of this issue.