Clarify what you can do with the .subscribe() Disposable

Author: tmewett

reactivex/observable/observable.py

@@ -88,7 +88,7 @@ def subscribe(
 
         Returns:
             Disposable object representing an observer's subscription to
-            the observable sequence.
+            the observable sequence. Call :code:`.dispose()` on it to unsubscribe.
         """
         if (
             isinstance(on_next, abc.ObserverBase)