JAVA-1410: Javadoc for legacy API

Made a first pass through the com.mongodb package and the existing API classes.  I've added some jira tickets for missing tests and code we might be able to delete, and updated the [Jira ticket](https://jira.mongodb.org/browse/JAVA-1410) with the classes that I couldn't document without help.
How about something like:

An iterator of documents that is possibly backed by a server-side cursor, which can be forcibly closed via the {@code close()} method.  Under normal circumstances, the cursor will be closed automatically when all the documents have been iterated, but care should be taken to manually close it in the face of early termination of the iterator.