doc: documented synthrecord known bugs

src/knot/modules/synthrecord/synthrecord.rst

@@ -85,6 +85,13 @@ Result:
    ;; ANSWER SECTION:
    1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.6.b.0.0.0.0.0.0.2.6.2.ip6.arpa. 400 IN PTR dynamic-2620-0-b61--1.test.
 
+Known bugs, limitations
+-----------------------
+
+The queries to the empty-non-terminals being parent to synthesized reverse records,
+e.g. `0.0.0.0.1.6.b.0.0.0.0.0.0.2.6.2.ip6.arpa. IN PTR` with previous example,
+are answered wrongly NXDOMAIN instead of correct NODATA.
+
 Module reference
 ----------------