Fix - docs: Correct Ajax documentation for how the return from
`ajax.data` as an object operates.

- The documention suggests that the object returned would be merged with
  the DataTables source data. That is quite possible and owuld actually
  simply the code slightly, but is less flexible than being able to use
  an entierly different object, as the code allowed. As such the
  documentation is updated to reflect how the code works

- This fixes issue #271