Cloud Connector: document backend field
What does this MR do and why?
In https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/9106 we added support for specifying the token aud
(ience) claim for Cloud Connector access tokens issued by CustomersDot.
We should document that this is done by defining the new backend
field introduced in https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/9634.
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