[api-ref] Add HTTP 501 for PUT min bw QoS rule
[api-ref] Add HTTP 501 for PUT min bw QoS rule

The change I477edb0ae35b385ac776a58195f22382e2fce4ed introduced a new
possible response code for PUT
/v2.0/qos/policies/{policy_id}/minimum_bandwidth_rules/{rule_id} API
but it missed to document it in the API ref. This patch amends the
documentation accordingly.

Closes-Bug: #1930283
Change-Id: I848e919d0a2e2c439aed7cd6e9ec0ab5102a43df

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib