[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.
Is this OK?

By definition, entry :math:`i, j` in a confusion matrix is
the number of observations actually in group :math:`i`, but
predicted to be in group :math:`j`. Here is an example::



On Wed, Oct 1, 2014 at 4:38 PM, jnothman <notifications@github.com> wrote:

> In doc/modules/model_evaluation.rst:
>
> >
> > -By definition, a confusion matrix :math:`C` is such that :math:`C_{i, j}` is
> > -equal to the number of observations known to be in group :math:`i` but
> > -predicted to be in group :math:`j`. Here an example of such confusion matrix::
> > +By definition, entry :math:`i, j` in confusion matrix :math:`C` is
>
> C is not used again and probably need not be defined.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/scikit-learn/scikit-learn/pull/3706/files#r18315427>.
>