Document list of valid ACLs

[dilyn-corner]

Currently the documentation makes reference to the ACLs of a credentials file, but doesn't provide a list of valid choices. Indeed, the only way most people can probably figure this out is after logging in by reviewing what their (presumably unrestricted) credentials give them access to.

I think a valid list is documented here.

It may be wise to include a link to this page and document them there?

@medubelko

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Reported from: https://documentation.ubuntu.com/snapcraft/stable/how-to/publishing/authenticate/