Update comment for postponing qual for SEMI JOIN

We added to the FIXME comment a link to an outstanding discussion on
gpdb-dev mailing list about whether or not it is correct for semi-join
to have quals deferred like this. If it is not, then the code should be
changed and the assert can go back to the same as upstream. If it is,
then the assert is right.
Co-authored-by: NMelanie Plageman <melanieplageman@gmail.com>

src/backend/optimizer/plan/initsplan.c

@@ -1021,6 +1021,11 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
 				 *
 				 * select * from A where exists (select * from B where A.i in
 				 * (select C.i from C where C.i = B.i));
+				 *
+				 * We are unsure if postponing quals past a semi-join is always
+				 * semantically correct, see discussion on mailing list here:
+				 * "Regarding postponing quals past an semi join"
+				 * https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/YHYNIUZnecI/Rlum0VD3FwAJ
 				 */
 				Assert(j->jointype == JOIN_INNER || j->jointype == JOIN_SEMI);
 				*postponed_qual_list = lappend(*postponed_qual_list, pq);