Improve description for nb parameter on head / tail method

[mthh]

The head/tail break method was contributed with the following description for the optional nb parameter : Optional number of class. If nb < natural breaks results, upper classes are merged. If nb > natural breaks results, nb is not used and the natural breaks results is returned..

But from git blame it looks like it's been overwritten by commit ee4ffb3 to replace it with "Number of classes desired". I think we should go back to the old description.