Clarify documentation

Discussion: https://www.postgresql.org/message-id/flat/20170618071607.GA16418%40nol.local

Author: petere

doc/src/sgml/ref/insert.sgml

@@ -227,8 +227,9 @@ INSERT INTO <replaceable class="PARAMETER">table_name</replaceable> [ AS <replac
         This clause is useful for example when copying values between tables.
         Writing <literal>INSERT INTO tbl2 OVERRIDING USER VALUE SELECT * FROM
         tbl1</literal> will copy from <literal>tbl1</literal> all columns that
-        are not identity columns in <literal>tbl2</literal> but will continue
-        the sequence counters for any identity columns.
+        are not identity columns in <literal>tbl2</literal> while values for
+        the identity columns in <literal>tbl2</literal> will be generated by
+        the sequences associated with <literal>tbl2</literal>.
        </para>
       </listitem>
      </varlistentry>