[MRG+1] PR for model_evaluation.rst changes to fix issue #3661, "Document negation of loss as scoring parameter"

* 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.
I think the comment column is clear. Thanks!

I wonder, however, whether it's now too wide and can be cut back by using 
```
.. currentmodule:: sklearn.metrics
```
and then using `:func:\`accuracy_score\`` etc rather than the prefixed forms...