subnet_service_types extension is not documented in api-ref

[Boden R]

The subnet_service_types extension is not documented in our api-ref:
- The subnet api-ref needs a subsection atop it describing the extn.
- Any attrs added by the extn need to be doc'd as params on the subnet api-ref.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/533412

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/533412
Committed: https://git.openstack.org/cgit/openstack/neutron-lib/commit/?id=2982196e5e6ac30420458271a25e48a38aede151
Submitter: Zuul
Branch: master

commit 2982196e5e6ac30420458271a25e48a38aede151
Author: Michal Kelner Mishali <email address hidden>
Date: Sun Jan 14 14:18:15 2018 +0200

    Document subnet_service_types extension in api-ref

    Adding sub-section for the extension, request params and
    json response

    Closes-Bug: 1733396

    Change-Id: I71e9add5a25129559aeda03027140d25cb934d84
    Signed-off-by: Michal Kelner Mishali <email address hidden>

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/neutron-lib 1.14.0 release.