Document how to specify singleBatch option with Find

Per behackett's comments in SPEC-941, it sounds like we should just document that a negative limit implies a single batch of results. Rather than allude to the extension's singleBatch option, I suggest we quote language from the server's cursor.limit() documentation:

A negative limit is similar to a positive limit but closes the cursor after returning a single batch of results. As such, with a negative limit, if the limited result set does not fit into a single batch, the number of documents received will be less than the specified limit. By passing a negative limit, the client indicates to the server that it will not ask for a subsequent batch via getMore.