Improve documentation of plugins.

Eigen/src/Core/MatrixBase.h

@@ -38,7 +38,7 @@
   * Note that some methods are defined in other modules such as the \ref LU_Module LU module
   * for all functions related to matrix inversions.
   *
-  * \param Derived is the derived type, e.g. a matrix type, or an expression, etc.
+  * \tparam Derived is the derived type, e.g. a matrix type, or an expression, etc.
   *
   * When writing a function taking Eigen objects as argument, if you want your function
   * to take as argument any matrix, vector, or expression, just let it take a
@@ -53,6 +53,9 @@
     }
   * \endcode
   *
+  * This class can be extended with the help of the plugin mechanism described on the page
+  * \ref TopicCustomizingEigen by defining the preprocessor symbol \c EIGEN_MATRIXBASE_PLUGIN.
+  *
   * \sa \ref TopicClassHierarchy
   */
 template<typename Derived> class MatrixBase