Update help command for ALTER TABLE for gpdb partition table

[my-ship-it]

                ereport(ERROR,
                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                         errmsg("addressing partition by RANK is no longer supported"),
                         errhint("Use partition name or FOR (<partition key value>) instead."),
                         parser_errposition(@3)));

We don't support RANK to address the partition, but the help command still shows it. We should avoid this usage in our official document or help command in formal product.