add RETURN VALUE to man page

buffer/buffer_getnewline_sa.3

@@ -20,5 +20,7 @@ token.
 On success, buffer_getnewline_sa returns 0.
 
 To read from a non-blocking socket, use buffer_getline_sa.
+.SH "RETURN VALUE"
+0 on success, -1 on error (setting \fIerrno\fR)
 .SH "SEE ALSO"
 buffer_get_token_sa(3), buffer(3)