V3 Cinder API doc, storage pools missing reponse parameters

[Martin Kopec]

Back-end storage pools, List all back-end storage pools is missing 2 response parameters:
- timestamp
- vendor_name

We found it when we were improving api unit tests here [2]

[1] https://docs.openstack.org/api-ref/block-storage/v3/?expanded=list-all-back-end-storage-pools-detail#back-end-storage-pools
[2] https://review.opendev.org/#/c/605917/10/tempest/tests/lib/services/volume/v3/test_scheduler_stats_client.py@33

-----------------------------------
Release: on 2020-05-17 02:43:19
SHA: a2fc5fa677b80a7a1a6c63275286dd93a7dbdf4d
Source: https://opendev.org/openstack/cinder/src/api-ref/source/v3/index.rst
URL: https://docs.openstack.org/api-ref/block-storage/v3/?expanded=list-all-back-end-storage-pools-detail