Document tempest APIs which plugins may use

Plugin authors can have a hard time determining which APIs are acceptable
to use, and which are not. We should provide an explicit list on the
plugin interface doc page.

Change-Id: I475a8ccc568077a4824fda0ddc20606a0ffabbc7
diff --git a/doc/source/plugin.rst b/doc/source/plugin.rst
index ad26741..9640469 100644
--- a/doc/source/plugin.rst
+++ b/doc/source/plugin.rst
@@ -15,10 +15,22 @@
 doing this is that the interfaces exposed by tempest are not considered stable
 (with the exception of configuration variables which ever effort goes into
 ensuring backwards compatibility). You should not need to import anything from
-tempest itself except where explicitly noted. If there is an interface from
-tempest that you need to rely on in your plugin it likely needs to be migrated
-to tempest.lib. In that situation, file a bug, push a migration patch, etc. to
-expedite providing the interface in a reliable manner.
+tempest itself except where explicitly noted.
+
+Stable Tempest APIs plugins may use
+-----------------------------------
+
+As noted above, several tempest APIs are acceptable to use from plugins, while
+others are not. A list of stable APIs available to plugins is provided below:
+
+* tempest.lib.*
+* tempest.config
+* tempest.test_discover.plugins
+
+If there is an interface from tempest that you need to rely on in your plugin
+which is not listed above, it likely needs to be migrated to tempest.lib. In
+that situation, file a bug, push a migration patch, etc. to expedite providing
+the interface in a reliable manner.
 
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