GraphQL relies heavily on self-documentation and introspection mechanism to provide API user with comprehensive knowledge about the API itself. Separate documentation is not necessary as every mature enough GraphQL client can query the API about available mutations and queries.
After obtaining admin token or access token and configuring your client to use them you should be able to explore the schema on your own using your preferred GraphQL API client.
Some examples of acclaimed GraphQL API clients include:
Centra also provides a web version of documentation if you would rather see it in your browser.