Bunch of fixes for bucket page.

* Move is_default() and bucket() up under the main BucketType objects
  section.
* Make datatype an attribute on BucketType.
* Reword section about deprecated search shortcuts.
* Add section about deprecated counters.
* Fix return-type documentation on multiget()
* Reword and complete documentation for new() and get().
* Add caveat to new_from_file()

Signed-off-by: Sean Cribbs <sean@basho.com>