Add notes about stream independence to horizon docs

[jedmccaleb]

If you are streaming the ledger closes and all the effects, will you get the ledger close event before or after all the effects come in? What I need is a way to know that I've gotten all the effects for a particular ledger so I can determine if and how I need to make a tx for the next ledger.

[nullstyle]

As in streaming this data across two separate TCP connections? There's no coordination between multiple streams... the order you will get responses on the client side will be undefined behavior.