added blurb about how only one Future can be returned

Author: Roee-87

documentation/guides/01_async.md

@@ -25,7 +25,7 @@ program first_mapping.aleo {
 }
 ```
 
-However, users can only call `transition` functions.  In order for the Future generated by an `async function` to be usable, it must be returned by a `transition` function.  Any `transition` that calls on an `async function` within its block must be annotated with the `async` keyword and must explicitly return a Future object.  Async transitions can return additional data types, including Records, along with a Future object.
+However, users can only call `transition` functions.  In order for the Future generated by an `async function` to be usable, it must be returned by a `transition` function.  Any `transition` that calls on an `async function` within its block must be annotated with the `async` keyword and must explicitly return a Future object.  Async transitions can return additional data types in a tuple, including Records, along with a `Future` object.  Only one `Future` can be returned.  If multiple types are returned, the `Future` must be the last type in the tuple.
 
 ```leo
 program first_mapping.aleo {