NXxas documentation improvement

[benajamin]

There was a question posted to the mailing list asking about how to implement the NXxas application definition. Itdescribes the use of 2 NXdetector instances of "incoming_beam" and "absorbed_beam". The NXdata group helpfully suggests linking to the "absorbed_beam" data field, but the required NXmonitor does not document any such suggestions even though it is clearly meant to be linked to the data field of the "incoming_beam" NXdetector. The doc strings for the NXmonitor should be updated to include suggested links.