[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:
- functions ending with ``_error`` or ``_loss`` return a
  value to minimize, the lower the better.  When converting
  into a scorer object using :func:`make_scorer`, set
  the ``greater_is_better`` parameter to False (True by default; see the
  parameter description below).



On Wed, Oct 1, 2014 at 4:25 PM, jnothman <notifications@github.com> wrote:

> In doc/modules/model_evaluation.rst:
>
> >
> >  - functions ending with ``_error`` or ``_loss`` return a
> > -  value to minimize (the lower the better).
> > +  value to minimize, the lower the better.  When converting
> > +  into a scorer object using :func:`make_scorer`, set
> > +  the ``greater_is_better`` parameter to False (True by default).
>
> Add here "and the output from cross validation will be negated to follow
> the convention that scorers return higher values for better models".
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/scikit-learn/scikit-learn/pull/3706/files#r18314944>.
>