Get the number of times this word occurs in the index

Number of times this word was found in all documents in this index. 

Note: To make WordListBuilder fast, it does not actually enumerate all of the references for each word, so in some cases the count can include artifacts of the indexing process such as removed or reindexed documents, which can make the reported count higher than the actual count in the index. Compressing an index will eliminate these extra references.