Update documentation: Default objective-weights and negative weights

[wmodes]

The current documentation says

By supplying weights via the --objective-weights flag, the user can control the relative importance of each column in the data.

However, there is no mention what the default weights are. If you are only setting a single column, what scale should I use to match the default weights? 0 to 1? 0-5? 0-10? 0-100?

Having this explicit would be helpful.

Also, are negative weights possible? This can be used to reverse the meaning of certain measures. Where diff tends to group heterogeneous criteria, a negative weight for diff would group homogeneous criteria.