Add documentation to rest_listen_uri #1847
This is also mentioned in the Graylog Collector documentation and there's even a troubleshooting item about this very issue: http://docs.graylog.org/en/1.3/pages/collector.html#unable-to-send-heartbeat
I'd prefer not to "pollute" the Graylog server config file with Graylog Collector specifics.
The unable to send heartbeat information is there but best case the configuration script that comes with the package should contain the information necessary to setup services. (barring the documentation site goes down or offline)
@lennartkoopmann 's addition is more descriptive but wordy, how about just:
Closes #1847 [ci skip]