Include link to bulk-helpers for `Document.to_dict()`

[kjaymiller]

The docs refer to use of elasticsearch.helpers.bulk as a way to manipulate many items.

This documentation change as a link to the documentation for elasticsearch.helpers.bulk this would provide an easy direction for questions concerning bulk operations such as https://github.com/elastic/elasticsearch-dsl-py/issues/1194#issuecomment-690751569.

Proposed Documentation Change

    def to_dict(self, include_meta=False, skip_empty=True):
        """
        Serialize the instance into a dictionary so that it can be saved in elasticsearch.
        :arg include_meta: if set to ``True`` will include all the metadata
            (``_index``, ``_id`` etc). Otherwise just the document's
            data is serialized. This is useful when passing multiple instances into the `elasticsearch-py`
            `bulk helper <https://elasticsearch-py.readthedocs.io/en/master/helpers.html#bulk-helpers>`
            ``elasticsearch.helpers.bulk`` 
        :arg skip_empty: if set to ``False`` will cause empty values (``None``,
            ``[]``, ``{}``) to be left on the document. Those values will be
            stripped out otherwise as they make no difference in elasticsearch.
        """

[ArtemBernatskyy]

@kjaymiller elasticsearch-dsl-py uses this under the hood

https://elasticsearch-py.readthedocs.io/en/7.10.0/helpers.html?highlight=actions#elasticsearch.helpers.bulk

[miguelgrinberg]

Considering that links to other documentation projects tend to break over time (as the one you included above now is) I think it is best to leave the reference as is. Note that putting elasticsearch.helpers.bulk on Google returns the correct and updated link to documentation in the first result.