[async.reqmts.executor.requirements] Reentrancy and dispatch()

[chriskohlhoff]

C++Std [reentrancy] says:

Except where explicitly specified in this standard, it is implementation-defined which functions in the Standard C++ library may be recursively reentered.

In the executor requirements, the intention is that the dispatch() function may be recursively reentered. A statement to this effect may need to be added to the requirements. (All dispatch() member functions provided by executors in the TS itself should then by implication allow reentrancy.)