DOC: Adding a few information for convolution

Code/BasicFilters/otbConvolutionImageFilter.h

@@ -43,7 +43,8 @@ namespace otb
  Default boundary conditions are zero flux Neumann boundary conditions.
  *
  * An optimized version of this filter using FFTW is available in the Orfeo ToolBox and
- * will significantly improves performances especially for large kernels.
+ * will significantly improves performances especially for large kernels 
+ * (see OverlapSaveConvolutionImageFilter).
  *
  * \sa Image
  * \sa Neighborhood

Code/BasicFilters/otbOverlapSaveConvolutionImageFilter.h

@@ -44,7 +44,8 @@ namespace otb
    * especially with large kernels.
    *
    * \note ITK must be set to use FFTW (double implementation) for this filter to work properly. If not, exception
-   *  will be raised at filter creation.
+   *  will be raised at filter creation. Install fftw and set the variable 
+   *  USE_FFTWD  to ON in ccmake.
    *
    * \sa ConvolutionImageFilter
    *