Add a comment to the example that pidfile_* functions can handle pfh being NULL.

MFC after:	3 days

lib/libutil/pidfile.3

@@ -150,6 +150,11 @@ if (pfh == NULL) {
 	}
 	/* If we cannot create pidfile from other reasons, only warn. */
 	warn("Cannot open or create pidfile");
+	/*
+	 * Eventhough pfh is NULL we can continue, as the other pidfile_*
+	 * function can handle such situation by doing nothing except setting
+	 * errno to EDOOFUS.
+	 */
 }
 
 if (daemon(0, 0) == -1) {