Fix omission in partitioning limitation documentation

UNIQUE and PRIMARY KEY constraints can be created on ONLY the
partitioned table.  We already had an example demonstrating that,
but forgot to mention it in the documentation of the limits of
partitioning.

Author: Laurenz Albe
Reviewed-By: shihao zhong, Shubham Khanna, Ashutosh Bapat
Backpatch-through: 12
Discussion: https://postgr.es/m/167299368731.659.16130012959616771853@wrigleys.postgresql.org

doc/src/sgml/ddl.sgml

@@ -4299,7 +4299,9 @@ ALTER INDEX measurement_city_id_logdate_key
         Using <literal>ONLY</literal> to add or drop a constraint on only
         the partitioned table is supported as long as there are no
         partitions.  Once partitions exist, using <literal>ONLY</literal>
-        will result in an error.  Instead, constraints on the partitions
+        will result in an error for any constraints other than
+        <literal>UNIQUE</literal> and <literal>PRIMARY KEY</literal>.
+        Instead, constraints on the partitions
         themselves can be added and (if they are not present in the parent
         table) dropped.
        </para>