Use sphinx-build -W in [docs] target

Even though [pbr] warnerrors = true is specified,
somehow sphinx warnings are not treated as errors.
If we use sphinx-build, -W option detects document warnings
and it leads to better documentation quality.

Fix a warning in the existing document too.

Change-Id: Icbb39686c7a24f0040d998fb14e14c63613da190

doc/source/devref/client_command_extensions.rst

@@ -70,6 +70,7 @@ neutronclient.extension entry_point
 
 To activate the commands in a specific extension module, add an entry in
 setup.cfg under neutronclient.extension. For example::
-  [entry_points]
-  neutronclient.extension =
-      fox_sockets = neutronclient.neutron.v2_0.contrib._fox_sockets
+
+    [entry_points]
+    neutronclient.extension =
+        fox_sockets = neutronclient.neutron.v2_0.contrib._fox_sockets

tox.ini

@@ -36,8 +36,7 @@ setenv =
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
 [testenv:docs]
-commands=
-    python setup.py build_sphinx
+commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html