GraphSON 3.0 doc updates

The upgrade notes indicate that GraphSON 3.0 is the default and even that 1.0, 2.0 configuration and example files have been dropped. However, the reference documentation mentions only 1.0,2.0 and does not discuss 3.0 at all. This is confusing.

Also, and maybe this should be a separate issue, the GraphSON 3.0 section in IO Reference should be standalone. Currently, the description is one sentence and to find out any details one has to go to the 2.0 section.