diff --git a/oslo/config/cfg.py b/oslo/config/cfg.py
index ece79160..98507b42 100644
--- a/oslo/config/cfg.py
+++ b/oslo/config/cfg.py
@@ -232,7 +232,16 @@ Option values may reference other values using PEP 292 string substitution::
                    help='Connection string for SQL database.'),
     ]
 
-Note that interpolation can be avoided by using `$$`.
+.. note::
+
+  Interpolation can be avoided by using `$$`.
+
+.. warning::
+
+  Interpolation using the values of options in groups is not yet
+  supported. The interpolated option must be in the DEFAULT group
+  (i.e., ``"$state_path"`` works but ``"$database.state_path"`` does
+  not).
 
 Special Handling Instructions
 -----------------------------
diff --git a/tox.ini b/tox.ini
index ebfdcfce..52c13ccf 100644
--- a/tox.ini
+++ b/tox.ini
@@ -19,6 +19,9 @@ commands =
 [testenv:venv]
 commands = {posargs}
 
+[testenv:docs]
+commands = python setup.py build_sphinx
+
 [flake8]
 show-source = True
 ignore = H305