[async.exec.ctx] Reentrancy and use_service/make_service

[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.

The intention is that both use_service and make_service may make nested calls (from the Service constructor) to use_service or make_service. Obviously these nested calls will require a different Service template argument. I am uncertain if calling these function templates with different template arguments counts as recursive reentrance, but if it does then we may need to add a sentence explicitly specifying that this is permitted.