Add documentation for config options

This adds a doc page for oslo.limit config options by auto-generating
them.

Change-Id: I49070fdbf2172f87bafd4d2bd08af282c9ef29a4
(cherry picked from commit 9f7a66a2)

doc/source/conf.py

@@ -21,7 +21,8 @@
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = [
     'openstackdocstheme',
-    'sphinxcontrib.apidoc'
+    'sphinxcontrib.apidoc',
+    'oslo_config.sphinxext',
 ]
 
 apidoc_module_dir = '../../oslo_limit'

doc/source/reference/index.rst

-========================
-oslo.limit API Reference
-========================
+=========
+Reference
+=========
 
 .. toctree::
+   :maxdepth: 2
+
+   opts
+
+API
+===
+
+.. toctree::
+   :maxdepth: 1
 
    api/modules

doc/source/reference/opts.rst

+=====================
+Configuration Options
+=====================
+
+oslo.limit uses oslo.config to define and manage configuration
+options to allow the deployer to control how an application uses the
+underlying quota limits and enforcement.
+
+.. show-options:: oslo.limit