Add documentation for extractCategories function

Author: wetneb

src/main/java/org/openrefine/extensions/commons/functions/ExtractCategories.java

@@ -47,14 +47,12 @@ public Object call(Properties bindings, Object[] args) {
 
     @Override
     public String getDescription() {
-        // TODO Auto-generated method stub
-        return null;
+        return "extracts the list of categories from the wikitext of a page";
     }
 
     @Override
     public String getReturns() {
-        // TODO Auto-generated method stub
-        return null;
+        return "arrays of strings";
     }
 
 }