doc: add note about fixing admin-only APIs without a microversion

This is a follow up to change I947e927802f755ccb25a91efd82cac895779d19e
to document the decision and agreements made in that change about fixing
obvious regression bugs in admin-only APIs without a microversion.

Change-Id: I4051cb465c509db63620ee727654f7c896fab1e8
Related-Bug: #1733886

doc/source/contributor/microversions.rst

@@ -216,6 +216,18 @@ A microversion is not needed in the following situation:
     returned with a 503 or 3xx response, so it may be removed without bumping
     the microversion.
 
+  - An obvious regression bug in an admin-only API where the bug can still
+    be fixed upstream on active stable branches. Admin-only APIs are less of
+    a concern for interoperability and generally a regression in behavior can
+    be dealt with as a bug fix when the documentation clearly shows the API
+    behavior was unexpectedly regressed. See [#f3]_ for an example. Intentional
+    behavior changes to an admin-only API *do* require a microversion, like the
+    :ref:`2.53 microversion <2.53-microversion>` for example.
+
+**Footnotes**
+
+.. [#f3] https://review.openstack.org/#/c/523194/
+
 In Code
 -------
 

nova/api/openstack/compute/rest_api_version_history.rst

@@ -633,6 +633,8 @@ There are two changes for the 2.51 microversion:
 Adds support for applying tags when creating a server. The tag schema is
 the same as in the `2.26`_ microversion.
 
+.. _2.53-microversion:
+
 2.53 (Maximum in Pike)
 ----------------------