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.
Now that I see you mention it below, I'm not sure it needs to be here, but do refer here to the custom scorer object discussion below.