Always recommend vertex data output in user docs

doc/default_files/richards-parameters.xml

@@ -52,9 +52,9 @@ adding an empty line, make text **bold** or ``monospaced``.
 
     <parameter name="vertexData">
       <definition> Plot vertex based (``true``) or cell-centered (``false``)
-       data into VTK files. Select vertex data when using the DG solver. System
-       tests and plotting functions (``dorie plot``) require cell-centered
-       data.
+       data into VTK files. Prefer vertex over cell data for full-precision
+       output. System tests and plotting functions (``dorie plot``) require
+       cell-centered data.
       </definition>
       <values> true, false </values>
       <suggestion> true </suggestion>