update PVC annotations secret supported in external-provisioner

[hoyho]

As mentioned in kubernetes-csi/external-provisioner#1148, external-provisioner didn't recognize per per volume secret tokens that reference to PVCs. This was fixed is master branch of external-provisioner. CSI document should be updated to advertise this change to users.
However, external-provisioner v4.0.2 hasn't published yet. This PR should wait until the new version is published.

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[jsafrane]

/lgtm
/approve

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This pull-request has been approved by: hoyho, jsafrane
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[jsafrane]

/assign @xing-yang
for approval