add a comment about sed and suid bits

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

editors/sed.c

@@ -1365,6 +1365,8 @@ int sed_main(int argc UNUSED_PARAM, char **argv)
 
 			/* Set permissions/owner of output file */
 			fstat(fileno(file), &statbuf);
+			/* chmod'ing AFTER chown would preserve suid/sgid bits,
+			 * but GNU sed 4.2.1 does not preserve them either */
 			fchmod(nonstdoutfd, statbuf.st_mode);
 			fchown(nonstdoutfd, statbuf.st_uid, statbuf.st_gid);
 			add_input_file(file);