[numerics] doxygen, add return type to NM_convert
@@ -713,8 +713,9 @@ extern "C"
/** Pass a NumericsMatrix through swig typemaps.
- * This is only usefull in python.
+ * This is only useful in python.
* \param A the matrix
+ * \return a NumericsMatrix
*/
static inline NumericsMatrix* NM_convert(NumericsMatrix* A)
{
