[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:

* for regression metrics only: whether the python function you provided
requires continuous decision
  certainties (``needs_threshold=True``).  The default value is
  False.


On Wed, Oct 1, 2014 at 4:30 PM, jnothman <notifications@github.com> wrote:

> In doc/modules/model_evaluation.rst:
>
> >
> > -* the function you want to use
> > -
> > -* whether it is a score (``greater_is_better=True``) or a loss
> > -  (``greater_is_better=False``),
> > -
> > -* whether the function you provided takes predictions as input
> > -  (``needs_threshold=False``) or needs confidence scores \
> > -  (``needs_threshold=True``)
> > +* whether the python function you provided requires continuous decision
>
> Perhaps we should note that this only applies for
> classification/clustering (i.e. discrete labeled tasks; regression metrics
> need not use needs_threshold afaik)
>
> —
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> <https://github.com/scikit-learn/scikit-learn/pull/3706/files#r18315107>.
>