Get the next document from the data source.

true if a document was obtained, or false if there are no more documents to process.

The dtSearch Engine will call GetNextDoc() during the index update to obtain pages to index. Each time GetNextDoc() is called, the Spider will either return the next page to index, or the Spider will return false indicating that there are no more pages to index.  

All of this happens automatically when you attach the HttpDataSource to the IndexJob and execute it, so there is no need for your code to interact with GetNextDoc().