Mention "futures" in rpc:async_call/5 documentation.

This addresses a comment in the Erlang Factory SF Bay Area 2014 talk on
the lack of discoverable language around "futures" in Erlang, "Catalyse
Change". The language around this concept, unfortunately, is fractured
but it seems many people are settling on _either_ 'promises' or
'futures'.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>