Made the specs more clear

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402378

dlib/svm/kkmeans_abstract.h

@@ -109,6 +109,8 @@ namespace dlib
         );
         /*!
             requires
+                - matrix_type and matrix_type2 must either be dlib::matrix objects or convertable to dlib::matrix
+                  via vector_to_matrix()
                 - matrix_type::type == sample_type  (i.e. matrix_type should contain sample_type objects)
                 - matrix_type2::type == sample_type (i.e. matrix_type2 should contain sample_type objects)
                 - initial_centers.nc() == 1         (i.e. must be a column vector)