Add warning about interpolating values from groups

Add a warning explaining that options in groups cannot be used with
the interpolation syntax.

Reformat note to use directive so it is highlighted.

Add "docs" section to tox.ini to make testing doc build easier.

Change-Id: If5106aa44d4c87abe84a7fb7a250541bba9f5068

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
 -----------------------------

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