r2_score - add more doctest examples (scikit-learn#7727)

Add simple examples showing perfect score, expected value score, and negative score.

sklearn/metrics/regression.py

@@ -438,7 +438,18 @@ def r2_score(y_true, y_pred,
  >>> y_pred = [[0, 2], [-1, 2], [8, -5]]
  >>> r2_score(y_true, y_pred, multioutput='variance_weighted') # doctest: +ELLIPSIS
  0.938...
-
+ >>> y_true = [1,2,3]
+ >>> y_pred = [1,2,3]
+ >>> r2_score(y_true, y_pred)
+ 1.0
+ >>> y_true = [1,2,3]
+ >>> y_pred = [2,2,2]
+ >>> r2_score(y_true, y_pred)
+ 0.0
+ >>> y_true = [1,2,3]
+ >>> y_pred = [3,2,1]
+ >>> r2_score(y_true, y_pred)
+ -3.0
  """
  y_type, y_true, y_pred, multioutput = _check_reg_targets(
  y_true, y_pred, multioutput)