Minor edit to master_modify_multiple_shards() examples

We currently have two examples in our documentation for ``master_modify_multiple_shards()``:

1. In the [reference section](https://docs.citusdata.com/en/v6.1/reference/user_defined_functions.html#master-modify-multiple-shards): This example issues a ``DELETE .. WHERE c_custkey > 500 AND c_custkey < 500``. Doesn't this return an empty set? Could we tweak this example to 200 and 500 (or something similar)?
2. In the [DML section](https://docs.citusdata.com/en/v6.1/dist_tables/dml.html#cross-shard-updates-and-deletion): This example issues a notice due to a restriction in Citus and it's a bit confusing. We have an issue open this issue in https://github.com/citusdata/citus/issues/923. Could we tweak this example to not include an ``IN`` statement?