[doc] Add examples on role overrides

At the moment it's not absolutely obvious on how to define
overrides for roles, or get them skipped from being cloned.

Adding examples may be useful for new users to have basic knowledge
on how to use custom content.

Depends-On: https://review.opendev.org/c/openstack/openstack-ansible-plugins/+/988143
Change-Id: Ia33cf31c9ae1e3bb90ce7dae14cc9209dc88cce7
Signed-off-by: Dmitriy Rabotyagov <dmitriy.rabotyagov@cleura.com>

doc/source/reference/configuration/extending-osa.rst

@@ -65,7 +65,7 @@ The environment variables set would use
 Adding new or overriding roles in your OpenStack-Ansible installation
 ---------------------------------------------------------------------
 
-By default OpenStack-Ansible uses its `ansible-role-requirements`_ file
+By default, OpenStack-Ansible uses its `ansible-role-requirements`_ file
 to fetch the roles it requires for the installation process.
 
 The roles will be fetched into the standard ``ANSIBLE_ROLES_PATH``,
@@ -96,9 +96,46 @@ in ``ansible-role-requirements.yml``, and roles with matching ``name`` key
 will override those in ``ansible-role-requirements.yml``. In case when
 ``src`` key is not defined bootstrap script will skip cloning such roles.
 
+Mapping of roles for override happens by the ``name`` key for the role,
+so it is important to keep the role name the same as in
+``ansible-role-requirements.yml`` if you want to update the source or version
+of the role. You have to execute ``./scripts/bootstrap-ansible.sh`` whenever you update
+``user-role-requirements.yml`` file to pull in the new versions.
+
 It is easy for a deployer to keep this file under their own version
 control and out of the OpenStack-Ansible tree.
 
+
+Override examples
+^^^^^^^^^^^^^^^^^
+
+* Update source location or the version of the role, defined in
+  ``ansible-role-requirements.yml``
+
+  .. code-block:: yaml
+
+    - name: os_ironic
+      scm: git
+      src: https://github.com/openstack/openstack-ansible-os_ironic
+      version: master
+
+* Use a patchset (unmerged, but existing change) for the role.
+  SHA and refspec can be taken from the Gerrit.
+
+  .. code-block:: yaml
+
+    - name: os_ironic
+      scm: git
+      src: https://opendev.org/openstack/openstack-ansible-os_ironic
+      refspec: refs/changes/09/835709/3
+      version: 6cf41cfa26128b64c422f22a103a8bc98b857f26
+
+* Skip role from being cloned/installed
+
+  .. code-block:: yaml
+
+    - name: os_ironic
+
 Adding new or overriding collections in your OpenStack-Ansible installation
 ---------------------------------------------------------------------------