Explain "shared state", and provide links to the explanation wherever it's
referenced.
We don't need the doc to enumerate 'R& future<R&>::get()' or 'void
future<void>::get()' separately from 'R future<R>::get()': all three
specializations are exactly as the reader expects, given the generic
template (unlike shared_future, where it's useful to spell them out).
Encapsulate some of the necessary redundancies between future and
shared_future documentation.
Explain the effect of promise::set_value() vs. promise::set_exception() on
each of [shared_]future::get(), get_exception_ptr(), wait(), wait_for() and
wait_until().
Add get_exception_ptr() method to shared_future exposition class.
Clarify distinction between shared_future move assignment vs. copy assignment.
Document that async() is now variadic. Document its StackAllocator overload,
and the effect of passing StackAllocator. Remove warning about launch policy
'deferred' since async() has no launch policy parameter.
a0490413cd