Clarify that VolumeContentSource is required for pre-populated volumes

[xing-yang]

In https://github.com/container-storage-interface/spec/blob/master/spec.md#createvolume, we have the following:

   // If specified, indicates that the volume is not empty and is
  // pre-populated with data from the specified source.
  // This field is OPTIONAL.
  VolumeContentSource content_source = 4;

The sentence "This field is OPTIONAL" is confusing. It means this field is not required in CreateVolumeResponse in general, however, it is required for pre-populated volume.

Some CSI driver developers found it confusing after we introduced a check to make sure content_source is set for pre-populated volume (kubernetes-csi/external-provisioner#283).

This should be clarified in the CSI spec.

[xing-yang]

/assign @xing-yang