Document OAuth2 Flow Grant Type configuration values

[abzolv]

Currently the Swagger UI JS uses non-standard values for the allowed OAuth2 flow grant types (accessCode, authorizationCode, clientCredentials, etc.), instead of the OAuth2 standards of authorization_code, client_credentials, etc.

Please document the fact that the grant names must be Camel Cased in the api_platform.yaml config file, or change the Symfony Bridge so that it will automatically camel case the oauth.flow value read in from the api_platform.yaml config file.

[soyuka]

these are documented in https://github.com/api-platform/docs/blob/2.5/core/configuration.md. Note that some improvements have been done recently in api-platform/core#3407 to help with openapi standard.