I think I figured it out. The Webhooks do not specify what has changed, just that something has changed.
After you receive a webhook you can query your documents by publication date (first_publication_date and last_publication_date ) and use the date.after predicate to get documents that have been published or changed since the previous time you received a webhook. With GraphQL you can use firstPublicationDate_after and lastPublicationDate_after arguments to query after a given date and time.