When a smart pg_ctl shutdown fails, mention -m fast as a tip.

src/bin/pg_ctl/pg_ctl.c

@@ -865,6 +865,9 @@ do_stop(void)
 			print_msg(_(" failed\n"));
 
 			write_stderr(_("%s: server does not shut down\n"), progname);
+			if (shutdown_mode == SMART_MODE)
+				write_stderr(_("TIP: the \"-m fast\" option immediately disconnects sessions rather than\n"
+							"waiting for session-initiated disconnection.\n"));
 			exit(1);
 		}
 		print_msg(_(" done\n"));
@@ -952,6 +955,9 @@ do_restart(void)
 			print_msg(_(" failed\n"));
 
 			write_stderr(_("%s: server does not shut down\n"), progname);
+			if (shutdown_mode == SMART_MODE)
+				write_stderr(_("TIP: the \"-m fast\" option immediately disconnects sessions rather than\n"
+							"waiting for session-initiated disconnection.\n"));
 			exit(1);
 		}