Add a doc example for yaml file overrides

Adds an example for yaml config file overrides and provides
clarification that the entire yaml file contents are overwritten
by any supplied override values, not just the added or changed values.

Change-Id: Ia64b6dceace4f33617736426c7818cfb6fc101e7
Partial-Bug: 1605758

doc/source/install-guide/app-advanced-config-override.rst

@@ -116,6 +116,52 @@ To assist you in finding the appropriate variable name to use for
 overrides, the general format for the variable name is
 ``<service>_policy_overrides``.
 
+Overriding YAML files
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You can override ``.yml`` file values by supplying replacement YAML content.
+
+.. note::
+
+   All default YAML file content will be completely overwritten by
+   the provided overrides, so the entire YAML source (both the existing
+   content and your changes) must be provided.
+
+For example, you can define a meter exclusion for all hardware items
+in the default content of ceilometer's ``pipeline.yml``:
+
+.. code-block:: yaml
+
+    sources:
+        - name: meter_source
+        interval: 600
+        meters:
+            - "!hardware.*"
+        sinks:
+            - meter_sink
+        - name: foo_source
+        value: foo
+
+You can accomplish this through the use of the following configuration
+entry in ``/etc/openstack_deploy/user_variables.yml``:
+
+.. code-block:: yaml
+
+    ceilometer_pipeline_yaml_overrides:
+      sources:
+          - name: meter_source
+          interval: 600
+          meters:
+              - "!hardware.*"
+          sinks:
+              - meter_sink
+          - name: source_foo
+          value: foo
+
+To assist you in finding the appropriate variable name to use for
+overrides, the general format for the variable name is
+``<service>_<filename>_<file extension>_overrides``.
+
 Currently available overrides
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~