Docs should contain required permissions in credentials for supported cloud storage providers

Where to find info upstream
https://github.com/vmware-tanzu/velero-plugin-for-aws#set-permissions-for-velero
https://github.com/vmware-tanzu/velero-plugin-for-gcp#set-permissions-for-velero
https://github.com/vmware-tanzu/velero-plugin-for-microsoft-azure#minimum-required-permissions

Additional permissions maybe required if you set backupImages: true
https://issues.redhat.com/browse/OADP-377.

Docs missing info
https://docs.openshift.com/container-platform/4.9/backup_and_restore/application_backup_and_restore/installing/installing-oadp-aws.html

• This currently contain velero-policy.json but doesn't say if this is the minimum required permissions.
• additional info is needed from https://issues.redhat.com/browse/OADP-377 as well.
  https://docs.openshift.com/container-platform/4.9/backup_and_restore/application_backup_and_restore/installing/installing-oadp-azure.html
  https://docs.openshift.com/container-platform/4.9/backup_and_restore/application_backup_and_restore/installing/installing-oadp-gcp.html