[doc][ZFSonLinux] Add 'manage snapshot' feature support

[OpenStack Infra]

https://review.openstack.org/337229
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/manila" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit d953baefcb2f22bbcdfdb992d3485a4dace67ca8
Author: Valeriy Ponomaryov <email address hidden>
Date: Mon Jul 4 15:08:03 2016 +0300

    [ZFSonLinux] Add 'manage snapshot' feature support

    Implement 'manage snapshot' feature in ZFSonLinux share driver.

    Prerequisites:
    - snapshot being managed should exist.

    Details:
    - snapshot being managed will be renamed by share driver based on
      taken ID.
    - size for managed snapshot will be taken either from
      REST API (--driver_options size=5) or
      from its "used" attribute.

    Implements BP zfsonlinux-manage-snapshot
    DocImpact

    Change-Id: If240ecde5676c334d39faaccd5703e93544aaa06
    Depends-On: Ifc762a882a78159adacfe168a4edbe824178301a

[Goutham Pacha Ravi (gouthamr)]

Hi Valeriy,

Anything to add to the devref within manila / configref within openstack-manuals? If not, we can mark this bug invalid.

[Tom Barron (tpb)]

Doc was included in commit d953baefcb2f22bbcdfdb992d3485a4dace67ca8 with the feature in question.