Document SHOW STATS FOR 'indexes'

[jacobmarble]

influxdata/influxdb#9766

https://docs.influxdata.com/influxdb/v1.5/administration/server_monitoring/#show-stats

Please document this new feature. It's important to note that SHOW STATS will not list index memory usage. 'indexes' must be explicitly requested because this can be an expensive metric to collect. We walk most of the index data structures recursively, so bigger indexes will take more time to count bytes.

[stevebang]

Fixed in recent PR -- https://docs.influxdata.com/influxdb/v1.6/query_language/spec/#show-stats