Update wording about information schema and name which views potentially

can have duplicates, per request from Tom.

doc/src/sgml/information_schema.sgml

@@ -32,6 +32,16 @@
    automatically-generated constraint names avoid duplicates in the
    same schema, but users can specify such duplicate names.
   </para>
+
+  <para>
+   This problem can appear when querying information schema views like
+   <literal>referential_constraints</>, <literal>check_constraints</>,
+   <literal>domain_constraints</>, and
+   <literal>check_constraint_routine_usage</>.  Some other views have
+   similar issues but contain the table name to help distinguish
+   duplicate rows, e.g. <literal>table_constraints</>,
+   <literal>constraint_table_usage</>, <literal>constraint_column_usage</>.
+  </para>
  </note>