postgres_fdw: Improve comment about handling of asynchronous requests.
We updated this comment in back branches (see commit
f6f61a4bd et al);
let's do so in HEAD as well for consistency.
Discussion: https://postgr.es/m/CAPmGK142V1kqDfjo2H%2Bb54JTn2woVBrisFq%2B%3D9jwXwxr0VvbgA%40mail.gmail.com