Limit results of ListMatchingWords or ListFieldValues to the documents in a SearchFilter

ListWords is not affected by SetFilter. Only ListMatchingWords and ListFieldValues limit their results to the items in the filter. 

When a SearchFilter is used, all counts returned by WordListBuilder are document counts. For example, if a word occurs 20 times in 7 documents, the count returned will be 7 for both GetNthWordCount and GetNthWordDocCount.