postgres_fdw: Improve comment about handling of asynchronous requests.

We updated this comment in back branches (see commit f6f61a4 et al);
let's do so in HEAD as well for consistency.

Discussion: https://postgr.es/m/CAPmGK142V1kqDfjo2H%2Bb54JTn2woVBrisFq%2B%3D9jwXwxr0VvbgA%40mail.gmail.com

Author: Etsuro Fujita

contrib/postgres_fdw/postgres_fdw.c

@@ -7270,14 +7270,16 @@ postgresForeignAsyncConfigureWait(AsyncRequest *areq)
 	{
 		/*
 		 * This is the case when the in-process request was made by another
-		 * Append.  Note that it might be useless to process the request,
-		 * because the query might not need tuples from that Append anymore.
-		 * If there are any child subplans of the same parent that are ready
-		 * for new requests, skip the given request.  Likewise, if there are
-		 * any configured events other than the postmaster death event, skip
-		 * it.  Otherwise, process the in-process request, then begin a fetch
-		 * to configure the event below, because we might otherwise end up
-		 * with no configured events other than the postmaster death event.
+		 * Append.  Note that it might be useless to process the request made
+		 * by that Append, because the query might not need tuples from that
+		 * Append anymore; so we avoid processing it to begin a fetch for the
+		 * given request if possible.  If there are any child subplans of the
+		 * same parent that are ready for new requests, skip the given
+		 * request.  Likewise, if there are any configured events other than
+		 * the postmaster death event, skip it.  Otherwise, process the
+		 * in-process request, then begin a fetch to configure the event
+		 * below, because we might otherwise end up with no configured events
+		 * other than the postmaster death event.
 		 */
 		if (!bms_is_empty(requestor->as_needrequest))
 			return;