Add information what +1 and -1 means (scikit-learn#8891)

Author: 1kastner

sklearn/svm/base.py

@@ -293,7 +293,7 @@ def _sparse_fit(self, X, y, sample_weight, solver_type, kernel,
     def predict(self, X):
         """Perform regression on samples in X.
 
-        For an one-class model, +1 or -1 is returned.
+        For an one-class model, +1 (inlier) or -1 (outlier) is returned.
 
         Parameters
         ----------