Provide a more informative deprecation message for `to_numpy_matrix` method

**Description**
The method [`to_numpy_matrix`](https://github.com/dwavesystems/dimod/blob/e125795f183954d60fd8c5316eacceac7b85e495/dimod/binary/binary_quadratic_model.py#L1841) raises a deprecation warning with the message `bqm.to_numpy_matrix() is deprecated since dimod 0.10.0`. This [note](https://github.com/dwavesystems/dimod/blob/6d1e4b1fccbfa556599533f934f15143db1a0ef6/dimod/core/bqm.py#L1063) in the `bqm.py` file suggests to use the `to_dense` method instead but that method doesn't seem to exist in the code base. It is not clear whether an alternative is available for `to_numpy_matrix`.

**Proposed Solution**
Suggest an alternative(if any) to the `to_numpy_matrix` method with a short code snippet along with the current message.