FTS - Add documentation for runtime query supervising capabilities

Search service users might want to monitor the currently running active queries in a node for FTS. This would help them gain insights into the ongoing slow-queries or other debugging purposes.

FTS has a new REST endpoint for serving this capability.

The users could use the `/api/query` endpoint for obtaining details about all the active queries in any FTS node in a cluster.

Example:

curl -XGET -H "Content-Type: application/json" -u<UserName:Password> http://localhost:9200/api/query

This endpoint takes an optional `longerThan` argument which can be used to filter the queries running beyond the given span of time.

curl -XGET -H "Content-Type: application/json" -uAdministrator:asdasd 'http://localhost:9200/api/query?longerThan=10s'

The users could use the `/api/query/index/{indexName}` endpoint for obtaining details about all the active queries against any given FTS index in the system. And the `longerThan` filter parameter is applicable here as well.

curl -XGET -H "Content-Type: application/json" -uAdministrator:asdasd 'http://localhost:9200/api/query/index/<indexName>?longerThan=1ms