Improve documentation for GROUP_BY operator wrt the value parameter

root@gradyhost1:~# curl gradyhost1:8080/v2/info | python -m json.tool
{
    "elected": true,
    "event_subscriber": null,
    "frameworkId": "fc4877a0-98ee-4e52-858d-afbe3b6a66e8-0000",
    "http_config": {
        "assets_path": null,
        "http_port": 8080,
        "https_port": 8443
    },
    "leader": "gradyhost1.eng.platformlab.ibm.com:8080",
    "marathon_config": {
        "checkpoint": true,
        "executor": "//cmd",
        "failover_timeout": 604800,
        "features": [],
        "framework_name": "marathon",
        "ha": true,
        "hostname": "gradyhost1.eng.platformlab.ibm.com",
        "leader_proxy_connection_timeout_ms": 5000,
        "leader_proxy_read_timeout_ms": 10000,
        "local_port_max": 20000,
        "local_port_min": 10000,
        "master": "zk://gradyhost1.eng.platformlab.ibm.com:2181,gradyhost2.eng.platformlab.ibm.com:2181,gradyhost3.eng.platformlab.ibm.com:2181/mesos",
        "mesos_leader_ui_url": "http://gradyhost1.eng.platformlab.ibm.com:5050/",
        "mesos_role": null,
        "mesos_user": "root",
        "reconciliation_initial_delay": 15000,
        "reconciliation_interval": 600000,
        "task_launch_timeout": 300000,
        "task_reservation_timeout": 20000,
        "webui_url": null
    },
    "name": "marathon",
    "version": "1.1.1",
    "zookeeper_config": {
        "zk": "zk://gradyhost1.eng.platformlab.ibm.com:2181,gradyhost2.eng.platformlab.ibm.com:2181,gradyhost3.eng.platformlab.ibm.com:2181/marathon",
        "zk_max_versions": 25,
        "zk_session_timeout": 10000,
        "zk_timeout": 10000
    }
}

root@gradyhost1:~# ps -ef | grep mesos-slave
root      1203 32490  0 08:25 pts/3    00:00:00 grep --color=auto mesos-slave
root     27575 27560  0 06:58 ?        00:00:11 mesos-slave --work_dir=/var/log/mesos --containerizers=mesos,docker --attributes=rack:rack-1 --master=zk://gradyhost1.eng.platformlab.ibm.com:2181,gradyhost2.eng.platformlab.ibm.com:2181,gradyhost3.eng.platformlab.ibm.com:2181/mesos

root@gradyhost2:~# ps -ef | grep mesos-slave
root      1169  1155  0 06:58 ?        00:00:09 mesos-slave --work_dir=/var/log/mesos --containerizers=mesos,docker --attributes=rack:rack-2 --master=zk://gradyhost1.eng.platformlab.ibm.com:2181,gradyhost2.eng.platformlab.ibm.com:2181,gradyhost3.eng.platformlab.ibm.com:2181/mesos
root      5034  4984  0 08:08 pts/2    00:00:00 grep --color=auto mesos-slave

root@gradyhost3:~# ps -ef | grep mesos-slave
root     14565 14551  0 06:58 ?        00:00:08 mesos-slave --work_dir=/var/log/mesos --containerizers=mesos,docker --attributes=rack:rack-3 --master=zk://gradyhost1.eng.platformlab.ibm.com:2181,gradyhost2.eng.platformlab.ibm.com:2181,gradyhost3.eng.platformlab.ibm.com:2181/mesos
root     18069 18049  0 08:08 pts/2    00:00:00 grep --color=auto mesos-slave

# curl -X POST -H "Content-type: application/json" gradyhost1:8080/v2/apps -d '{
    "id": "test-group-by",
    "cmd": "sleep 6000",
    "instances": 6,
    "constraints": [["rack", "GROUP_BY", "2"]]
}'

# curl gradyhost1:8080/v2/apps/test-group-by | jq '. | .app.tasks[].host'
"gradyhost1.eng.platformlab.ibm.com"
"gradyhost3.eng.platformlab.ibm.com"
"gradyhost2.eng.platformlab.ibm.com"
"gradyhost3.eng.platformlab.ibm.com"
"gradyhost2.eng.platformlab.ibm.com"
"gradyhost1.eng.platformlab.ibm.com"