Fix awkward wording for rpc acknowledgement

From the API reference for acknowledged rpc:

1440 Within user-progress of the
1441 recipient’s master persona, the result from invoking func(args...) will placed
1442 in a rpc_ff destined to the initiating rank. In user-progress of the initiating
1443 persona, it will be deserialized and fulfilled into the final future

This loose use of "a rpc_ff" to specify the acknowledgement semantics here is awkward and somewhat sloppy.

upcxx::rpc_ff() requires a func argument, but the contents of that func (and whatever function arguments it takes) are not formally clarified. Also normal upcxx:rpc_ff() is only serviced by user-level progress of the master persona, but this acknowledgement message is actually serviced by the persona who initiated the rpc. That's two fundamental ways in which this acknowledgement differs and has to be clarified.

It would be better to just directly state the acknowledgement semantics without reference to a library function which is not really the same.

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