Additional documentation for features

Classification/include/CGAL/Classification/Feature_base.h

@@ -33,6 +33,13 @@ namespace Classification {
 
   \brief Abstract class describing a classification feature that
   associates a scalar value to each item of the classification input.
+
+  User-defined features must inherit this class. Feature objects are
+  generated by `CGAL::Classifier` objects by the method
+  `CGAL::Classifier::add_feature()`. For this method to work, the
+  first argument of the feature's constructors must be of type
+  `ItemRange&` (where `ItemRange` is the first template argument of
+  `CGAL::Classifier`).
 */