mesos events should have a more descriptive help message

[ssk2]

Currently:

(env)➜  mesos-cli git:(master) ✗ mesos events --help
usage: mesos-events [-h] [-v] [-s SLEEP_INTERVAL] [-q]

observe events from the cluster

optional arguments:
  -h, --help            show this help message and exit
  -v, --version         show program's version number and exit
  -s SLEEP_INTERVAL, --sleep-interval SLEEP_INTERVAL
                        Sleep approximately N seconds between iterations
                        (default: 5)
  -q                    Suppresses printing of headers when multiple tasks are
                        being examined (default: False)

A small amount more detail would be useful, I'm curious as to:

• Where are these events collected from? (Presumably primarily the cluster.)
• It looks like this streams events from the cluster, which wasn't obvious to me. Perhaps "Streams events from the cluster"
• What do iterations refer to? What is an iteration?