Add warning on output file size when using subsampling

[ci skip]

doc/default_files/parameters.xml

@@ -66,7 +66,8 @@ adding an empty line, make text **bold** or ``monospaced``.
         supports bilinear triangulations and displays higher-order solutions
         inappropriately. Use level 0 for order 1, and level N+1 for order N.
         For level > 0, the printed grid does not resemble the actual grid.
-        This parameter defaults to 0 if not given in the config file.
+        This parameter defaults to 0 if not given in the config file. Notice
+        that subsampling significantly increases the output file size!
       </definition>
       <values> int </values>
       <suggestion> 0 </suggestion>