Document Stream Spec vs Observable Spec quirks

[brianegan]

This includes:

• Dart Sync vs Async Streams vs / in combination with Rx Schedulers as we introduce that concept
• Return type of first, last, etc returns Future not an Observable
• Error contract in Observable (closes stream) vs Dart (does not by default)

[lestathc]

Suggestion, may want to mention:
StreamController onListen side effect triggering logic is different than Observable onSubscribe (for create operator).

Well, Observable onSubscribe is not officially documented in ReactiveX that it must be called every time Observable is subscribed, but it is in all existing rx implementations.

But for broadcast stream, the onListen is only be called at the first time it is subscribed, so that it is equal to observable.publish().autoConnect(1). (For non-broadcast stream, it is fine since it can not be subscribed multiple times)