Update Direct Message events' expansions in docs/expansions_and_fields.rst

Update Direct Message events' expansions in docs/expansions_and_fields.rst

For consistency, this section should also be taken from the description for the expansions parameter for endpoints that return the object type, e.g. https://developer.twitter.com/en/docs/twitter-api/direct-messages/lookup/api-reference/get-dm_conversations-with-participant_id-dm_events for DM events.

Future work: improve this section of the documentation in the future to include information like this from https://developer.twitter.com/en/docs/twitter-api/expansions

Co-authored-by: Harmon <Harmon758@gmail.com>