Clearly identify C++ futures/promises

[Nenad Vukicevic]

I noticed in the section on futures/promises:

5.5. Execution model
Futures have the capability to express dataflow/task-based programming, and other software frameworks provide thread-level parallelism by considering each callback to be a task that can be run in an arbitrary worker thread. This is not the case in UPC++.

As UPC++ futures were already introduced earlier, the sentence should start like this:

C++ std::futures ...

Otherwise it is confusing.

[Dan Bonachea]

I disagree - the introductory phrase in question is referring to futures as the general computer-science abstraction, not the futures from any particular system (eg in particular, not just C++). The two subsequent sentences expand and clarify.