cinder storageclass, zone algorithm if not specified

update aws+gce availability texts, the algorithm is not random

Author: zetaab

Diff for: docs/concepts/storage/persistent-volumes.md

@@ -443,7 +443,7 @@ parameters:
 ```
 
 * `type`: `io1`, `gp2`, `sc1`, `st1`. See [AWS docs](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSVolumeTypes.html) for details. Default: `gp2`.
-* `zone`: AWS zone. If not specified, a random zone from those where Kubernetes cluster has a node is chosen.
+* `zone`: AWS zone. If not specified, volumes are generally round-robin-ed across all active zones where Kubernetes cluster has a node.
 * `iopsPerGB`: only for `io1` volumes. I/O operations per second per GiB. AWS volume plugin multiplies this with size of requested volume to compute IOPS of the volume and caps it at 20 000 IOPS (maximum supported by AWS, see [AWS docs](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSVolumeTypes.html). A string is expected here, i.e. `"10"`, not `10`.
 * `encrypted`: denotes whether the EBS volume should be encrypted or not. Valid values are `"true"` or `"false"`. A string is expected here, i.e. `"true"`, not `true`.
 * `kmsKeyId`: optional. The full Amazon Resource Name of the key to use when encrypting the volume. If none is supplied but `encrypted` is true, a key is generated by AWS. See AWS docs for valid ARN value.
@@ -462,7 +462,7 @@ parameters:
 ```
 
 * `type`: `pd-standard` or `pd-ssd`. Default: `pd-standard`
-* `zone`: GCE zone. If not specified, a random zone in the same region as controller-manager will be chosen.
+* `zone`: GCE zone. If not specified, volumes are generally round-robin-ed across all active zones where Kubernetes cluster has a node.
 
 #### Glusterfs
 
@@ -526,7 +526,7 @@ parameters:
 ```
 
 * `type`: [VolumeType](http://docs.openstack.org/admin-guide/dashboard-manage-volumes.html) created in Cinder. Default is empty.
-* `availability`: Availability Zone. Default is empty.
+* `availability`: Availability Zone. If not specified, volumes are generally round-robin-ed across all active zones where Kubernetes cluster has a node.
 
 #### vSphere