postgres_fdw: Update comments in make_new_connection().

Expand the comment about the parallel_commit option to mention that the
default is false.

Also, since the comment about alteration of the keep_connections option,
which was located above the expanded comment, holds true for the
parallel_commit option, rewrite it to reflect this, and move it to after
the expanded comment.

Follow-up for commit 04e706d42.

Discussion: https://postgr.es/m/CAPmGK16Kg2Bf90sqzcZ4YM5cN_G-4h7wFUS01qQpqNB%2B2BG5_w%40mail.gmail.com

contrib/postgres_fdw/connection.c

@@ -318,14 +318,15 @@ make_new_connection(ConnCacheEntry *entry, UserMapping *user)
 	 * open even after the transaction using it ends, so that the subsequent
 	 * transactions can re-use it.
 	 *
-	 * It's enough to determine this only when making new connection because
-	 * all the connections to the foreign server whose keep_connections option
-	 * is changed will be closed and re-made later.
-	 *
 	 * By default, all the connections to any foreign servers are kept open.
 	 *
 	 * Also determine whether to commit (sub)transactions opened on the remote
-	 * server in parallel at (sub)transaction end.
+	 * server in parallel at (sub)transaction end, which is disabled by
+	 * default.
+	 *
+	 * Note: it's enough to determine these only when making a new connection
+	 * because these settings for it are changed, it will be closed and
+	 * re-made later.
 	 */
 	entry->keep_connections = true;
 	entry->parallel_commit = false;