[async.dispatch] uses DEDUCED without completion signatures

[jwakely]

[async.reqmts.async.return.type] defines DEDUCED as a type that depends on Signature, but then [async.dispatch] doesn't specify a completion signature for the dispatch/post/defer overloads that use DEDUCED. It can be inferred to be void() from the Effects clause, but it's not specified explicitly.

[chriskohlhoff]

Added completion signatures in a7f3a348a8fccd8b367761e879d32b956236310e.

N.B. dispatch() and defer() don't strictly meet all the requirements of asynchronous operations. I wonder if we need a note to that effect?

[chriskohlhoff]

Add note as described