Add terseClusterInfo to the REST API docs

We are documenting the following REST API that users can use to identify which node is the orchestrator. pools/default/terseClusterInfo.

This REST API returns the following information:

$ curl -sX GET -u Administrator:password 'http://hostname:8091/pools/default/terseClusterInfo' | jq .

{

  "clusterUUID":"929bd61a4b16ae5555940c6720e8f312",

  "orchestrator":"n_1@127.0.0.1",

  "isBalanced":true,

  "clusterCompatVersion":"7.0"

}

Cluster UUID is self explanatory.

"orchestrator" refers to the node which is currently believed to be the orchestrator by the node against which the REST API is invoked. There are a number of caveats that users of this REST API need to bear in mind:

1. the value will be "undefined" if the node is unaware of the existence of an orchestrator for whatever reason (this is the initial value for instance)
2. other nodes may believe there to be a different orchestrator and in fact if a network is partitioned this will happen
3. the orchestrator may change at any time. In fact by the time you have received the value it may have already changed.

"isBalanced" is true if this nodes understanding of the cluster is that a rebalance is not required. (I.e. data is distributed evenly and the topology aware services are in balance.)

Cluster compat version is this nodes understanding of the compat version a cluster is running at. Cluster compat version are the minimum compat version of all nodes in the cluster. E.g. if a cluster has 9 7.0 nodes and 1 6.5 node, the compat version is 6.5.