Lang???.gf: exclude Documentation from present mode

The Documentation module is excluded from present mode in abstract/Lang.gf,
so it needs to be excluded in the corresponding concrete syntaxes too
to avoid problems.

src/dutch/LangDut.gf

@@ -3,6 +3,6 @@
 concrete LangDut of Lang = 
   GrammarDut,
   LexiconDut 
-  ,DocumentationDut
+  ,DocumentationDut --# notpresent
   ,ConstructionDut
 ;