[DOC]Add cast_rules_to_readonly to share instances

[OpenStack Infra]

https://review.openstack.org/419163
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 0970eb6e3aee8f77cfb0e6660490c2bfd84ab1d5
Author: Goutham Pacha Ravi <email address hidden>
Date: Thu Dec 1 16:50:39 2016 +0530

    Add cast_rules_to_readonly to share instances

    - Add Database migration to introduce the column on the
      share instances model.
    - Set the field to True if creating read-only secondary
      replicas, unset while promoting them.
    - Set the field to True if drivers don't support writable access
      to migrating shares, or if using host assisted migration.
      Unset if migration fails, or is canceled.
    - Expose the field via share-instances and share-replicas
      APIs to administrators.

    Supporting read only-access rules is part of the minimum
    driver requirements in manila.

    APIImpact
    DocImpact

    Implements: bp fix-and-improve-access-rules

    Co-Authored-By: Rodrigo Barbieri <email address hidden>

    Change-Id: Ie8425f36f02cbcede0aaa9f3fe1f5f3cf23df8b8

[Goutham Pacha Ravi (gouthamr)]

The change https://review.opendev.org/419163 introduced a property on the share instance model called "cast_rules_to_readonly". This property is available in the JSON response of APIs:

 GET /v2/{project_id}/share-replicas/{share_replica_id}
 GET /v2/{project_id}/share-replicas/
 GET /v2/{project_id}/share-instances/{instance-id}
 GET /v2/{project_id}/share-instances

The API Reference has been updated for share replica APIs in https://review.opendev.org/494143/ and for share instance APIs in https://review.opendev.org/648005

No further documentation is necessary.