Clarify Bamboo.Endpoint's uniqueness

[cwegrzyn]

This makes explicit that each Bamboo instance needs a unique Endpoint address.

We figured this out the hard way after setting the endpoint to a DNS name that round-robined all of the bamboo servers. Only one instance would ever receive an update from Marathon, and so the bamboo instances all end up out-of-sync.

Happy to swap in any other/better language-- but hopefully this will save folks from making the mistake we did.

[j1n6]

Thank you, any doc improvements are always appreciated.