Update description of API and access

[MyPyDavid]

this original sentence is from 7 years ago...

rdmo-docs/docs/administration/api.md

Line 24 in 6ed8f27

Only `Superusers` are allowed to access the API. You can check if your user has the required permissions by having a look into the user's details. If the user does not have the Superuser status, any API request will fail, producing the status code 403.

[jochenklar]

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