Made a comment more clear

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402259

dlib/is_kind.h

@@ -47,7 +47,7 @@ namespace dlib
     struct is_matrix : public default_is_kind_value  
     {
         /*!
-            - if (T is an implementation of matrix/matrix_abstract.h) then
+            - if (T is some kind of matrix expression from the matrix/matrix_abstract.h component) then
                 - is_matrix<T>::value == true
             - else
                 - is_matrix<T>::value == false