[CBM] Docs don't make it clear which GCP credentials are required

GCP offers a variety of different authentication mechanisms (https://cloud.google.com/docs/authentication). For cbbackupmgr, we specifically require  the Application Default Credentials (https://cloud.google.com/docs/authentication/provide-credentials-adc), and this is not mentioned in the docs (https://docs.couchbase.com/server/7.2/backup-restore/cbbackupmgr-cloud.html#gcp).  We should make it clearer in the documentation.