updated AS:Callbacks docs, variable namings

- Update the doc to match the new behavior on  ba552764344bc0a3c25b8576ec11f127ceaa16d
- Rename the define_callbacks params to `names`
  - in order to match the naming conventions for `get_callbacks` and `set_callbacks` at https://github.com/rails/rails/blob/master/activesupport/lib/active_support/callbacks.rb#L736-743
  - `define_callbacks` just register names(events), not define the real callback functions.