Docs: Add more explanation about the complexity of networkConnector elements

When explaining this to the PE team, I was reminded of many many gotchas lying
in wait. This makes the doc more wordy, unfortunately, but I think it adds
enough explanation that it'll be easier to get these right.
Merge pull request #109 from nfagerlund/activemq_network_connectors_again

Docs: Add more explanation about the complexity of networkConnector elements