document error case

io/iob_new.3

@@ -18,5 +18,8 @@ APIs like FreeBSD's sendfile.  The file contents will always be sent in
 a way that allows the operating systems to perform zero copy TCP, and
 the buffers will always be sent using as few syscalls as possible and
 avoiding unnecessary copying (using writev).
+.SH "RETURN VALUE"
+iob_new returns a pointer to an I/O batch data structure.  If there was
+a memory allocation error, it returns NULL instead.
 .SH "SEE ALSO"
 iob_reset(3), iob_send(3), iob_addbuf(3), iob_adds_free(3), iob_addfile(3)