This also adds more documentation and corrects the release notes
for these breaking changes. These modules will now only be included
in the full builds of Select2, which has been noted in a few places.
Now if the old `initSelection` method is used, a warning will be
triggered in the console informing the user that it has now been
changed to `DataAdapter.current`, and they should create a custom
data adapter instead. We will still only call `initSelection` once,
and then default back to the old data adapter for calls after that.
This allows for mostly transparent support between versions.
This adds tests to ensure that `initSelection` operates the same as
it previously did, but now patches the `current` method of the
default data adapter.
