Made spec more clear

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403231

dlib/svm/feature_ranking_abstract.h

@@ -28,6 +28,8 @@ namespace dlib
     );
     /*!
         requires
+            - sample_matrix_type == a matrix or something convertible to a matrix via vector_to_matrix()
+            - label_matrix_type  == a matrix or something convertible to a matrix via vector_to_matrix()
             - is_binary_classification_problem(samples, labels) == true
             - kc.train(samples(0)) must be a valid expression.  This means that
               kc must use a kernel type that is capable of operating on the