Add Venn diagram showing taxonomy of traits and capabilities

This was put together while working on the mechanism for converting
driver capabilities to traits in I15364d37fb7426f4eec00ca4eaf99bec50e964b6:

    https://review.openstack.org/538498

and may help other developers working on this area in the future.

Change-Id: I395e386ee713769d4c105be0dd6e821382945866

doc/source/admin/configuration/schedulers.rst

@@ -1473,6 +1473,8 @@ use :command:`nova-manage cell_v2 create_cell`. See the
 :ref:`man-page-cells-v2` man page for details on command usage.
 
 
+.. _compute-capabilities-as-traits:
+
 Compute capabilities as traits
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -1540,3 +1542,7 @@ There are some rules associated with capability-defined traits.
    library.
 
 .. _os-traits: https://opendev.org/openstack/os-traits/src/branch/master/os_traits/compute
+
+:ref:`Further information on capabilities and traits
+<taxonomy_of_traits_and_capabilities>` can be found in the
+:doc:`Technical Reference Deep Dives section </reference/index>`.

doc/source/reference/update-provider-tree.rst

@@ -209,3 +209,30 @@ would become:
   def update_provider_tree(self, provider_tree, nodename):
       provider_tree.add_traits(
           nodename, 'HW_CPU_X86_AVX', 'HW_CPU_X86_AVX2', 'CUSTOM_GOLD')
+
+.. _taxonomy_of_traits_and_capabilities:
+
+Taxonomy of traits and capabilities
+-----------------------------------
+
+There are various types of traits:
+
+- Some are standard (registered in
+  `os-traits <https://docs.openstack.org/os-traits/latest/>`_); others
+  are custom.
+
+- Some are owned by the compute service; others can be managed by
+  operators.
+
+- Some come from driver-supported capabilities, via a mechanism which
+  was `introduced <https://review.opendev.org/538498>`_ to convert
+  them to standard traits on the compute node resource provider.  This
+  mechanism is :ref:`documented in the configuration guide
+  <compute-capabilities-as-traits>`.
+
+This diagram may shed further light on how these traits relate to each
+other and how they are managed.
+
+.. figure:: /figures/traits-taxonomy.svg
+   :width: 800
+   :alt: Venn diagram showing taxonomy of traits and capabilities