Document distributed lock configuration

We're now relying on distributed locks in order to prevent race
conditions that affect instance failover handling.

This change documents the required configuration updates and
prerequisites.

Change-Id: Ic08b28ac16faea673838aa2dc05c1dd70806280e

doc/source/configuration/index.rst

@@ -187,6 +187,34 @@ configured. For this, check the `nova PCI passthrough configuration guide`__.
 __ https://docs.openstack.org/nova/queens/admin/pci-passthrough.html
 
 
+Distributed locking configuration
+---------------------------------
+
+In order to avoid race conditions, our driver relies on distributed locks. A
+distributed lock backend such as etcd, mysql or a file share will have to be
+configured.
+
+The following configuration will use etcd 3 as a lock backend:
+
+.. code-block:: ini
+
+    [coordination]
+    backend_url = etcd3+http://etcd_address:2379
+
+.. note::
+
+   The ``etcd3gw`` python package is required when using etcd 3. This does not
+   apply to the v2 etcd API, which may be requested through
+   ``etcd://etcd_address:2379``.
+
+In order to use a file share, set the following:
+
+.. code-block:: ini
+
+    [coordination]
+    backend_url = file:////share_addr/share_name
+
+
 Configuration options
 ---------------------
 

doc/source/install/prerequisites.rst

@@ -233,6 +233,15 @@ Cluster Driver to be usable, the Hyper-V Cluster nodes will have to be joined
 to an Active Directory and a Microsoft Failover Cluster. The nodes in a
 Hyper-V Cluster must be identical.
 
+In order to avoid race conditions, our driver relies on distributed locks. A
+distributed lock backend such as etcd, mysql or a file share will have to be
+configured.
+
+For more details about available distributed lock backends, check the
+`list of drivers supported by tooz`__.
+
+__ https://docs.openstack.org/tooz/latest/user/drivers.html
+
 
 Guarded Host configuration (Shielded VMs)
 -----------------------------------------