prow.sh: avoid AllAlpha=true

It turned out to not work. Instead of reverting the commit which
introduced this, let's better document this explicitly.

prow.sh

@@ -267,9 +267,9 @@ configvar CSI_PROW_E2E_ALPHA "$(get_versioned_variable CSI_PROW_E2E_ALPHA "${csi
 # the failing test for "latest" or by updating the test and not running
 # it anymore for older releases.
 configvar CSI_PROW_E2E_ALPHA_GATES_1_13 'VolumeSnapshotDataSource=true,BlockVolume=true,CSIBlockVolume=true' "alpha feature gates for Kubernetes 1.13"
-configvar CSI_PROW_E2E_ALPHA_GATES_1_14 'VolumeSnapshotDataSource=true' "alpha feature gates for Kubernetes 1.14"
+configvar CSI_PROW_E2E_ALPHA_GATES_1_14 'VolumeSnapshotDataSource=true,ExpandCSIVolumes=true' "alpha feature gates for Kubernetes 1.14"
 # TODO: add new CSI_PROW_ALPHA_GATES entry for future Kubernetes releases
-configvar CSI_PROW_E2E_ALPHA_GATES_LATEST 'AllAlpha=true' "alpha feature gates for latest Kubernetes"
+configvar CSI_PROW_E2E_ALPHA_GATES_LATEST 'AllAlpha=true,ExpandCSIVolumes=true' "alpha feature gates for latest Kubernetes"
 configvar CSI_PROW_E2E_ALPHA_GATES "$(get_versioned_variable CSI_PROW_E2E_ALPHA_GATES "${csi_prow_kubernetes_version_suffix}")" "alpha E2E feature gates"
 
 # Some tests are known to be unusable in a KinD cluster. For example,