Document negation of loss as scoring parameter (fixes #3661)

* doc/modules/model_evaluation.rst in the MattpSoftware fork contains my changes for issue #3661.

* issue-3661/model_evaluation_copy_edited.rst also fixes typos I found when reading the document.

* issue-3661/model_evaluation_edit_notes.txt has the explanation for the changes in model_evaluation_copy_edited.rst, and flags a few other other items for questions.
Perhaps we should note that this only applies for classification/clustering (i.e. discrete labeled tasks; regression metrics need not use `needs_threshold` afaik)